Guide to electrical safety

A Guide to
Electrical Safety
N. C. Department of Labor
Occupational Safety and Health Division
1101 Mail Service Center
Raleigh, NC 27699- 1101
Cherie Berry
Commissioner of Labor
18
N. C. Department of Labor
Occupational Safety and Health Program
Cherie Berry
Commissioner of Labor
OSHA State Plan Designee
Allen McNeely
Deputy Commissioner for Safety and Health
Kevin Beauregard
Assistant Deputy Commissioner for Safety and Health
Edward E. Lewis
Reviewer
Acknowledgments
A Guide to Electrical Safety was prepared by Ed Mendenhall of Mendenhall Technical Services with additional materi-als
provided by N. C. Department of Labor employee Dwight Grimes. The information in this guide was updated in
2008.
This guide is intended to be consistent with all existing OSHA standards; therefore, if an area is considered by the
reader to be inconsistent with a standard, then the OSHA standard should be followed.
To obtain additional copies of this guide, or if you have questions about North Carolina occupational safety and health stan-dards
or rules, please contact:
N. C. Department of Labor
Education, Training and Technical Assistance Bureau
1101 Mail Service Center
Raleigh, NC 27699- 1101
Phone: ( 919) 807- 2875 or 1- 800- NC- LABOR
____________________
Additional sources of information are listed on the inside back cover of this guide.
____________________
The projected cost of the NCDOL OSH program for federal fiscal year 2009– 2010 is $ 17,534,771. Federal funding provides approximately 30 percent ($ 5,180,700) of
this fund.
Revised 2/ 08
Contents
Part Page
Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1iiv
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ivi1
2 Fundamentals of Electricity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii14
3 Arc Flash/ NFPA 70E . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii11
4 Branch Circuit Wiring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii13
5 Branch Circuit and Equipment Testing . . . . . . . . . . . . . . . . . . . . . . . . . ii21
6 Voltage Detector Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii24
7 Ground Fault Circuit Interrupters . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii26
8 Common Electrical Deficiencies . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii43
9 Inspection Guidelines/ Checklist . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii57
10 Safety Program Policy and Procedures . . . . . . . . . . . . . . . . . . . . . . . . . ii59
iii
Foreword
Everyone from office clerks to farmers work around electricity on a daily basis. Our world is filled with overhead
power lines, extension cords, electronic equipment, outlets and switches. Our access to electricity has become so common
that we tend to take our safety for granted. We forget that one frayed power cord or a puddle of water on the floor can
take us right into the electrical danger zone.
A Guide to Electrical Safety can help electricians, plant maintenance personnel and many others review safe proce-dures
for electrical work. It also covers the main U. S. Occupational Safety and Health Administration standards concern-ing
electrical safety on the job.
In North Carolina, state inspectors enforce the federal laws through a state plan approved by the U. S. Department of
Labor. The N. C. Department of Labor is charged with this mission. NCDOL enforces all current OSHA standards. It
offers many educational programs to the public and produces publications, including this guide, to help inform people
about their rights and responsibilities.
When reading this guide, please remember the NCDOL mission is greater than enforcement of regulations. An equally
important goal is to help citizens find ways to create safer workplaces. A Guide to Electrical Safety can help you make
and keep your workplace free of dangerous electrical hazards.
Cherie Berry
Commissioner of Labor
v
1
1
Introduction
Electricity is the modern version of the genie in Aladdin’s lamp. When electricity is safely contained in an insulated
conductor, we normally cannot see, smell, taste, feel or hear it. It powers an endless list of laborsaving appliances and
life- enhancing and support systems that have become such an assumed part of our lives that we give little thought to its
potential for causing harm. Many myths and misstatements about electrical action are accepted as fact by many people.
The National Institute for Occupational Safety and Health ( NIOSH) conducted a study of workplace electrocutions that
revealed the following information about workers who were electrocuted:
• The average age was 32.
• 81 percent had a high school education.
• 56 percent were married.
• 40 percent had less than one year of experience on the job to which they were assigned at the time of the fatal acci-dent.
• 96 percent of the victims had some type of safety training, according to their employers.
This information reminds us that more effective training and education must be provided to employees if we are to
reduce workplace electrocution hazards. Employees should receive initial training then refresher electrical hazard recog-nition
training on an annual basis.
In addition to the shock and electrocution hazards, electricity can also cause fires and explosions. According to the U. S.
Consumer Product Safety Commission, an estimated 169,000 house fires of electrical origin occur each year, claiming
1,100 lives and injuring 5,600 people. Property losses from fires begun by electricity are estimated at $ 1.1 billion each
year. The safe use and maintenance of electrical equipment at work ( and at home) will help prevent fire and physical injury.
This guide provides a clear understanding of electrical action and its control in the workplace environment. This infor-mation
will enable you to recognize electrical hazards in the workplace as well as provide information on their control
and/ or elimination. The guide does not qualify a person to work on or near exposed energized parts. Training
requirements for “ qualified” persons ( those permitted to work on or near exposed energized parts) are detailed in
29 CFR 1910.332( b)( 3). Also, 29 CFR 1910.399, Definitions Applicable to Subpart S gives a definition of “ qualified
person.” The guide will, however, enhance your ability to find and report electrical deficiencies in need of a quali-fied
person’s attention.
Dangers of Electricity
Whenever you work with power tools or on electrical circuits, there is a risk of electrical hazards, especially electrical
shock. Anyone can be exposed to these hazards at home or at work. Workers are exposed to more hazards because job-sites
can be cluttered with tools and materials, fast- paced, and open to the weather. Risk is also higher at work because
many jobs involve electric power tools.
Electrical trades workers must pay special attention to electrical hazards because they work on electrical circuits.
Coming in contact with an electrical voltage can cause current to flow through the body, resulting in electrical shock and
burns. Serious injury or even death may occur. As a source of energy, electricity is used without much thought about the
hazards it can cause. Because electricity is a familiar part of our lives, it often is not treated with enough caution. As a
result, an average of one worker is electrocuted on the job every day of every year. Electrocution is the third leading cause
of work- related deaths among 16- and 17- year- olds, after motor vehicle deaths and workplace homicide. Electrocution is
the cause of 12 percent of all workplace deaths among young workers. 1
____________
1 Castillo D. N. [ 1995]. NIOSH Alert: Preventing Death and Injuries of Adolescent Workers. Cincinnati, Ohio: U. S. Department of Health and Human Services, Public
Health Service, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, DHHS ( NIOSH) Publication No. 95- 125.
• Electrical shock causes injury or death!
• current— the movement of electrical charge
• voltage— a measure of electrical force
• circuit— a complete path for the flow of current
• You will receive a shock if you touch two wires at different volt-ages
at the same time.
This industry guide offers discussion on a variety of topics as pertained
to electrical hazards. There are four main types of electrical injuries: elec-trocution
( death due to electrical shock), electrical shock, burns and
falls. The guide discusses the dangers of electricity, electrical shock and
the resulting injuries. It describes the various electrical hazards. The guide includes a sample plan ( Safety Model) or
approach to address these hazards in a later section. ( This sample model/ approach is also useful with other hazards.) You
will learn about the Safety Model, as an important tool for recognizing, evaluating and controlling hazards. The guide
includes important definitions and notes are shown throughout. It emphasizes practices that will help keep you safe and
free of injury. It also includes case studies about real- life deaths to give you an idea of the hazards caused by electricity.
How Is an Electrical Shock Received?
An electrical shock is received when electrical current passes through the body. Current will pass through the body in a
variety of situations. Whenever two wires are at different voltages, current will pass between them if they are connected.
Your body can connect the wires if you touch both of them at the same time. Current will pass through your body.
• ground— a physical electrical connection to the earth
• energized ( live, “ hot”)— similar terms meaning that a voltage is present that can cause a
current, so there is a possibility of getting shocked
In most household wiring, the black wires and the red wires are at 120 volts. The white wires are
at 0 volts because they are connected to ground. The connection to ground is often through a con-ducting
ground rod driven into the earth. The connection can also be made through a buried metal
water pipe. If you come in contact with an energized black wire— and you are also in contact with
the neutral white wire— current will pass through your body. You will receive an electrical shock.
• conductor— material in which an electrical current moves easily
• neutral— at ground potential ( 0 volts) because of a connection to ground
If you are in contact with a live wire or any live component
of an energized electrical device— and also in contact with any
grounded object— you will receive a shock. Plumbing is often
grounded. Metal electrical boxes and conduit are grounded.
Your risk of receiving a shock is greater if you stand in a
puddle of water. But you don’t even have to be standing in
water to be at risk. Wet clothing, high humidity and perspiration
also increase your chances of being shocked. Of course, there is
always a chance of shock, even in dry conditions. You can even
receive a shock when you are not in contact with an electrical
ground. Contact with both live wires of a 240- volt cable will
deliver a shock. ( This type of shock can occur because one live
wire may be at + 120 volts while the other is at – 120 volts dur-ing
an alternating current cycle— a difference of 240 volts.) You
can also receive a shock from electrical components that are not
grounded properly. Even contact with another person who is
receiving an electrical shock may cause you to be shocked.
2
Electrical work can be deadly if not done safely.
Wires carry current.
Metal electrical boxes should
be grounded to prevent shocks.
Black and red wires are
usually energized, and
white wires are usually
neutral.
• You will receive a shock if you touch a live wire and are grounded at the same time.
• When a circuit, electrical component or equipment is energized, a potential shock haz-ard
is present.
Summary
You will receive an electrical shock if a part of your body completes an electrical circuit by
touching a live wire and an electrical ground, or touching a live wire and another wire at a differ-ent
voltage.
3
Always test a circuit
to make sure it is
de- energized before
working on it.
2
Fundamentals of Electricity
A review of the fundamentals of electricity is necessary to an understanding of some common myths and misstate-ments
about electricity. First we must review Ohm’s Law and understand the effects of current on the human body. Basic
rules of electrical action will enhance your ability to analyze actual or potential electrical hazards quickly. This informa-tion
will also enable you to understand other important safety concepts such as reverse polarity, equipment grounding,
ground fault circuit interrupters, double insulated power tools, and testing of circuits and equipment.
Ohm’s Law
There are three factors involved in electrical action. For electrons to be activated or caused to flow, those three factors
must be present. A voltage ( potential difference) must be applied to a resistance ( load) to cause current to flow when there
is a complete loop or circuit to and from the voltage source. Ohm’s Law simply states that 1 volt will cause a current of 1
ampere to flow through a resistance of 1 ohm. As a formula this is stated as follows:
Voltage ( E) = Current ( I) X Resistance ( R).
We will be concerned about the effects of current on the human body, so the formula relationship we will use most will
be I = E/ R. When you analyze reported shock hazards or electrical injuries, you should look for a voltage source and a
resistance ( high or low) ground loop. The human body is basically a resistor and its resistance can be measured in ohms.
Figure 1 depicts a body resistance model. The resistive values are for a person doing moderate work. An increase in per-spiration
caused from working at a faster work pace would decrease the resistance and allow more current to flow.
As an example, let’s use the hand to hand resistance of the body model, 500 + 500
= 1,000 ohms. Using I = E/ R, I = 120/ 1,000 ( assuming a 120 volt AC ( alternating
current) power source) or 0.120 amps. If we multiply 0.120 amps by 1,000 ( this con-verts
amps to milliamps), we get 120 milliamps ( mA) which we will refer to in
Figure 2. If a person were working in a hot environment, and sweating, the body
resistance could be lowered to a value of 500 ohms. Then the current that could flow
through the body would equal I = 120/ 500 or 0.240 amps. Changing this to mil-liamps,
1,000 X 0.240 = 240 mA. This means that we have doubled the hazard to the
body by just doing our job.
This can be explained by looking at Figure 2. Figure 2 plots the current flowing
through the chest area and the time it takes to cause the heart to go into ventricular
fibrillation
( arrhythmic
heartbeat).
Using the
example of
the body
resistance at
1,000 ohms allowing 120 mA to flow ( follow
the dark line vertically from 120 mA to the
shaded area, then left to the time of 0.8 seconds),
you can see that it would only take 0.8 seconds
to cause electrocution. When the body resistance
is 500 ohms, at 240 mA it would only take 0.2
seconds to cause electrocution. Variable condi-tions
can make common- use electricity ( 110
volts, 15 amps) fatal.
4
Figure 1
Human Body Resistance Model
10.0
6.0
2.0
1.0
0.6
.2
.1
.06
.02
.01
Time In Seconds
‘ Let Go’
Range
Maximum Permitted
By UL For Class A GFCI
Electrocution Threshold
For Typical Adult
0 20 40 60 80 100 120 140 160 180 200 220 240 260
Current In Milliamperes
Figure 2
Electrical Current ( AC) Versus the Time It Flows Through the Body
Current and Its Effect on the Human Body
Based on the research of Professor Dalziel of the University of California, Berkeley, the effect of 60 Hz ( cycles per
second) of alternating current on the human body is generally accepted to be as follows:
• 1 milliamp ( mA) or less— no sensation— not felt ( 1,000 milliamps equal 1 amp)
• 3 mA or more painful shock
• 5 mA or more— local muscle contractions— 50 percent cannot let go
• 30 mA or more— breathing difficult— can cause unconsciousness
• 50– 100 mA— possible heart ventricular fibrillation
• 100– 200 mA— certain heart ventricular fibrillation
• 200 mA or more— severe burns and muscular contractions— heart more apt to stop than fibrillate
• Over a few amps— irreversible body damage
Thus, we can see that there are different types of injuries that electricity can cause. At the 20 to 30 mA range a form of
anoxia ( suffocation) can result. This could happen in a swimming pool where there is a ground loop present ( the drain at the
bottom of the pool) if a faulty light fixture or appliance is dropped into the water. Current would flow from the light fixture
to the drain, using the water as the conducting medium. Any person swimming through the electrical field created by the
fault current would be bathed in potential difference, and the internal current flow in the body could paralyze the breathing
mechanism. This is why it is very important to keep all portable electrical appliances away from sinks, tubs and pools.
Ventricular fibrillation generally can occur in the range of 50 to 200 mA. Ventricular fibrillation is the repeated, rapid,
uncoordinated contractions of the ventricles of the heart resulting in the loss of synchronization between the heartbeat and
the pulse beat. Once ventricular fibrillation occurs, death can ensue in a few minutes. Properly applied CPR ( cardiopul-monary
resuscitation) techniques can save the victim until emergency rescue personnel with a defibrillator arrive at the
scene. Workers in the construction trades and others working with electrical power tools should receive CPR training.
Above a few amperes, irreversible body damage can occur. This condition is more likely to occur at voltages above
600 volts AC. For example, if a person contacted 10,000 volts, I = 10,000/ 1,000 = 10 amps. This amount of current would
create a great amount of body heat. Since the body consists of over 60 percent water, the water would turn to steam at a
ratio of approximately 1 to 1,500. This would cause severe burns or exploding of body parts. These are the types of
injuries that you would normally associate with electric power company workers. They can also occur, however, when
people accidentally let a television or radio antenna contact an uninsulated power line. Accidents involving mobile verti-cal
scaffolding or cranes booming up into power lines can cause these types of injuries or fatalities.
The route that the current takes through the body affects the degree of injury. If the current passes through the chest
cavity ( e. g., left hand to right hand), the person is more likely to receive severe injury or electrocution; however, there
have been cases where an arm or leg was burned severely when the extremity came in contact with the voltage and the
current flowed through a portion of the body without going through the chest area of the body. In these cases the person
received a severe injury but was not electrocuted.
Typical 120 Volt AC System
At some time in your life, you
may have received an electrical
shock. Figure 3 illustrates a typical
120 volt AC system. Somewhere
near your home or workplace there
is a transformer with wires going
between the transformer and the ser-vice
entrance panel ( SEP). In small
establishments and homes, the SEP
may also contain circuit breakers or
fuses to protect the circuits leaving
the SEP. Typical overcurrent protec-tion
for these circuits would be 15 or
20 amps. This protection is designed
5
120 Volts
120 Volts
Neutral
Grounding Electrode
Equipment- grounding
Conductor ( green or bare)
Utility
Supply
Service
Ground
Circuit Breaker
Grounded
Conductor
( white or gray)
“ Hot” Conductor
( black or red)
Primary Lines
2.4– 13 kV
Figure 3
AC Systems— Contact With “ Hot” Conductor
for line ( hot) to line ( grounded conductor) faults that would cause current greater than 15 or 20 amps to flow. If a person
accidentally contacted the “ hot��� conductor while standing on the ground with wet feet ( see Figure 3), a severe shock could
result. Current could flow through the body and return to the transformer byway of the “ ground loop” path. Most electrical
shocks result when the body gets into a ground loop and then contacts the “ hot” or ungrounded conductor. If you analyze
electrical shock incidents, look for these two factors: a ground loop and a voltage source.
We normally think of ground as the earth beneath our feet. From an electrical hazard standpoint, ground loops are all
around us. A few ground loops that may not be under our feet include metal water piping, metal door frames in newer
building construction, ventilation ducts, metal sinks, metal T- bars holding ceiling acoustical panels, wet or damp concrete
floors and walls, grounded light fixtures, and grounded power tools/ appliances. When you are using or working around
electrical equipment, be alert to these and other ground loops. The person shown in Figure 3 could have isolated the
ground loop by standing on insulated mats or dry plywood sheets. Wearing dry synthetic soled shoes would also have iso-lated
the ground loop.
The utility supply ground and the grounding electrode conductor are system safety grounds. These grounds protect the
users of electrical equipment in case of lightning storms and in instances where high voltage lines accidentally fall on
lower voltage lines. These system safety grounds are not designed for individual safety. Actually, they are a hazard to the
individual in that it is very easy to get into a ground loop, and once into a ground loop, you only need one fault path to the
hot conductor before shock or injury can result.
Four Principles of Electrical Action
Knowing the basic principles of electrical action will help you understand and evaluate electrical shock hazards. These
principles and an explanation for each are as follows:
1. Electricity does not “ spring” into action until current flows.
2. Current will not flow until there is a loop ( intentionally or accidentally) from the voltage source to a load and back
to the source.
3. Electrical current always returns to the voltage source ( transformer) that created it.
4. When current flows, energy ( measured in watts) results.
Explanation for Principle 1
A person can contact voltage and not be shocked if there is high resistance in the loop. In Figure 3, the person is stand-ing
on the ground and touching the 120 volt conductor. That would cause a shock and make your hair stand on end. If that
same person were standing on insulated mats or wore shoes with insulated soles, the person would not be shocked even
though there was 120 volts in his or her body. This explains why a person can be working outside with a defective power
tool and not receive a shock when the ground is very dry or the person is isolated from a ground loop by plywood. That
same person with the same power tool could change work locations to a wet area, then receive a shock when contacting a
ground loop of low resistance. As previously stated, 3 mA or more can cause painful shock. Using Ohm’s Law I = E/ R,
120 volts and 3 mA, we can calculate how much resistance would allow 3 mA of current to flow. R = E/ I or 120/ 0.003 or
R = 40,000 ohms. Any ground loop resistance of less than 40,000 ohms would allow a shock that could be felt. This prin-ciple
can also explain why birds sitting on a power line are not electrocuted. Their bodies would receive voltage, but cur-rent
would not flow since another part of their body is not in contact with a ground loop.
Explanation for Principle 2
For current to flow, a complete loop must be established from the voltage source to the person and back to the voltage
source. In Figure 3, the loop is through the person’s hand touching a 120 volt conductor, through the body to ground and
then through the grounding electrode and back to the transformer secondary through the neutral conductor. Once that loop
is established and becomes less than 40,000 ohms, a shock or serious injury can result. If the loop can be interrupted, as
noted in Principle 1, then current will not flow. These two principles give you a common sense way to figure out how and
why someone received a shock and the action that should be taken to prevent future shocks of the same type.
Explanation for Principle 3
Electric current always seeks to return to the transformer that created it. Current will also take all resistive paths to
return to the transformer that created it. Since the voltage source has one wire already connected to ground ( Figure 3),
6
contact with the “ hot” wire provides a return path for current to use. Other ground loop paths in the workplace could
include metal ducts, suspended ceiling T- bars, water pipes and other similar ground loops.
Explanation for Principle 4
This principle explains the shock and injury to the human body that current can do. The higher the voltage involved,
the greater the potential heat damage to the body. As previously mentioned, high voltage can cause high current flow
resulting in severe external and internal body damage. Remember that the flow of current causes death or injury; voltage
determines how the injury or death is effected.
Some Misconceptions About Electrical Action
Americans use more electrical power per person than do individuals of any other country in the world, but that does
not mean that we have a better understanding of electricity. Some common misconceptions about electrical actions are
addressed and corrected in the following discussion.
“ If an Appliance or Power Tool Falls Into Water, It Will Short Out���
When an appliance falls into a tub or container of water, it will not short out. In fact, if the appliance switch is “ on,” the
appliance will continue to operate. If the appliance has a motor in it, the air passage to keep the motor cool will be water
cooled. Unfortunately, that same air passage, when wet, will allow electricity to flow outside the appliance if a current
loop is present ( such as a person touching the metal faucet and reaching into the water to retrieve a hair dryer). The cur-rent
loop due to the water resistance will be in the 100 to 300 mA range, which is considerably less than the 20,000+ mA
needed to trip a 20 amp circuit breaker. Since an appliance will not short out when dropped in a sink or tub, no one should
ever reach into the water to retrieve an appliance accidentally dropped there. The water could be electrified, and a person
touching a grounded object with some other part of the body could receive a serious shock depending on the path the cur-rent
takes through the body. The most important thing to remember is that appliances do not short out when dropped or
submerged in water.
“ Electricity Wants to Go to the Ground”
Sometimes editors of motion films about electrical safety make the statement that “ electricity wants to go to the
ground.” There are even books published about electrical wiring that contain the same statement. As previously stated,
electricity wants to return to the transformer that created it, and the two conductors that were designed to carry it safely
are the preferred route it takes. Whenever current goes to ground or any other ground loop, it is the result of a fault in the
appliance, cords, plugs or other source.
“ It Takes High Voltage to Kill; 120 Volts AC Is Not Dangerous”
Current is the culprit that kills. Voltage determines the form of the injury. Under the right conditions, AC voltage as
low as 60 volts can kill. At higher voltages the body can be severely burned yet the victim could live. Respect all AC
voltages, high or low, as having the potential to kill.
“ Double Insulated Power Tools Are Doubly Safe and Can Be Used in Wet and Damp Locations”
Read the manufacturer’s operating instructions carefully. Double insulated power tools are generally made with materi-al
that is nonconductive. This does give the user protection from electrical faults that occur within the insulated case of
the appliance. However, double insulated power tools can be hazardous if dropped into water. Electrical current can flow
out of the power tool case into the water. Remember that double insulated power tools are not to be used in areas where
they can get wet. If conditions or situations require their use under adverse conditions, use GFCI ( ground fault circuit
interrupter) protection for the employee.
• ampere ( amp)— the unit used to measure current
• milliampere ( milliamp or mA)— 1/ 1,000 of an ampere
• shocking current— electrical current that passes through a part of the body
• You will be hurt more if you can’t let go of a tool giving a shock.
• The longer the shock, the greater the injury.
7
Dangers of Electrical Shock
The severity of injury from electrical shock depends on the amount of electrical current and the length of time the current
passes through the body. For example, 1/ 10 of an ampere ( amp) of electricity going through the body for just 2 seconds is
enough to cause death. The amount of internal current a person can withstand and still be able to control the muscles of the
arm and hand can be less than 10 milliamperes ( milliamps or mA). Currents above 10 mA can paralyze or “ freeze” muscles.
When this “ freezing” happens, a person is no longer able to release a tool, wire or other object. In fact, the electrified object
may be held even more tightly, resulting in longer exposure to the shocking current. For this reason, hand- held tools that give
a shock can be very dangerous. If you can’t let go of the tool, current continues through your body for a longer time, which
can lead to respiratory paralysis ( the muscles that control breathing cannot move). You stop breathing for a period of time.
People have stopped breathing when shocked with currents from voltages as low as 49
volts. Usually, it takes about 30 mA of current to cause respiratory paralysis.
Currents greater than 75 mA cause ventricular fibrillation ( very rapid, ineffective
heartbeat). This condition will cause death within a few minutes unless a special device
called a defibrillator is used to save the victim. Heart paralysis occurs at 4 amps, which
means the heart does not pump at all. Tissue is burned with currents greater than 5 amps. 2
Table 1 shows what usually happens for a range of currents ( lasting one second) at
typical household voltages. Longer exposure times increase the danger to the shock vic-tim.
For example, a current of 100 mA applied for 3 seconds is as dangerous as a cur-rent
of 900 mA applied for a fraction of a second ( 0.03 seconds). The muscle structure
of the person also makes a difference. People with less muscle tissue are typically
affected at lower current levels. Even low voltages can be extremely dangerous because
the degree of injury depends not only on the amount of current but also on the length of
time the body is in contact with the circuit.
LOW VOLTAGE DOES NOT MEAN LOW HAZARD!
Table 1
Effects of Electrical Current* on the Body3
8
Defibrillator in use.
Current Reaction
1 milliamp Just a faint tingle.
5 milliamps Slight shock felt. Disturbing, but not painful. Most people can let go.
However, strong involuntary movements can cause injuries.
6– 25 milliamps ( women)† Painful shock. Muscular control is lost. This is the range where " freezing currents" start.
9– 30 milliamps ( men) It may not be possible to let go.
5– 150 milliamps Extremely painful shock, respiratory arrest ( breathing stops), severe muscle contractions.
Flexor muscles may cause holding on; extensor muscles may cause intense pushing
away. Death is possible.
1,000– 4,300 milliamps Ventricular fibrillation ( heart pumping action not rhythmic) occurs. Muscles contract;
( 1– 4.3 amps) nerve damage occurs. Death is likely.
10,000 milliamps Cardiac arrest and severe burns occur. Death is probable.
( 10 amps)
15,000 milliamps Lowest overcurrent at which a typical fuse or circuit breaker opens a circuit!
( 15 amps)
* Effects are for voltages less than about 600 volts. Higher voltages also cause severe burns.
† Differences in muscle and fat content affect the severity of shock.
____________
2 Lee R. L. [ 1973]. Electrical Safety in Industrial Plants. Am Soc Safety Eng J18( 9): 36- 42.
3 USDOL [ 1997]. Controlling Electrical Hazards. Washington, D. C.: U. S. Department of Labor, Occupational Safety and Health Administration.
Sometimes high voltages lead to additional injuries. High voltages can cause violent muscular contractions. You may
lose your balance and fall, which can cause injury or even death if you fall into machinery that can crush you. High volt-ages
can also cause severe burns ( as seen on photos later in this and other sections).
• High voltages cause additional injuries.
At 600 volts, the current through the body may be as great as 4 amps, causing damage to internal organs such as the
heart. High voltages also produce burns. In addition, internal blood vessels may clot. Nerves in the area of the contact
point may be damaged. Muscle contractions may cause bone fractures from either the contractions themselves or from
falls.
• Higher voltages can cause larger currents and more severe shocks.
A severe shock can cause much more damage to the body than is visible. A person may suffer internal bleeding and
destruction of tissues, nerves, and muscles. Sometimes the hidden injuries caused by electrical shock result in a delayed
death. Shock is often only the beginning of a chain of events. Even if the electrical current is too small to cause injury,
your reaction to the shock may cause you to fall, resulting in bruises, broken bones, or even death.
• Some injuries from electrical shock cannot be seen.
The length of time of the shock greatly affects the amount of injury. If the shock is short in duration, it may only be
painful. A longer shock ( lasting a few seconds) could be fatal if the level of current is high enough to cause the heart to go
into ventricular fibrillation. This is not much current when you realize that a small power drill uses 30 times as much cur-rent
as what will kill. At relatively high currents, death is certain if the shock is long enough. However, if the shock is
short and the heart has not been damaged, a normal heartbeat may resume if contact with the electrical current is eliminat-ed.
( This type of recovery is rare.)
• The greater the current, the greater the shock.
• Severity of shock depends on voltage, amperage, and resistance.
• Resistance— a material’s ability to decrease or stop electrical current.
• Ohm— unit of measurement for electrical resistance.
• Lower resistance causes greater currents.
• Currents across the chest are very dangerous.
The amount of current passing through the body also affects the severity of an electrical shock. Greater voltages pro-duce
greater currents. There is greater danger from higher voltages. Resistance hinders current. The lower the resistance
( or impedance in AC circuits), the greater the current will be. Dry skin may have a resistance of 100,000 ohms or more.
Wet skin may have a resistance of only 1,000 ohms. Wet working conditions or broken skin will drastically reduce resis-tance.
The low resistance of wet skin allows current to pass into the body more easily and give a greater shock. When
more force is applied to the contact point or when the contact area is larger, the resistance is lower, causing stronger
shocks.
The path of the electrical current through the
body affects the severity of the shock. Currents
through the heart or nervous system are most
dangerous. If you contact a live wire with your
head, your nervous system will be damaged.
Contacting a live electrical part with one hand-while
you are grounded at the other side of
your body- will cause electrical current to pass
across your chest, possibly injuring your heart
and lungs.
• NEC— National Electrical Code— a com-prehensive
listing of practices to protect
workers and equipment from electrical
hazards such as fire and electrocution
There have been cases where an arm or leg
is severely burned by high- voltage electrical
9
Power drills use 30 times as much
current as what will kill.
current to the point of coming off, and the victim is not electrocuted. In these cases, the current passes through only a part
of the limb before it goes out of the body and into another conductor. Therefore, the current does not go through the chest
area and may not cause death, even though the victim is severely disfigured. If the current does go through the chest, the
person will almost surely be electrocuted. A large number of serious electrical injuries involve current passing from the
hands to the feet. Such a path involves both the heart and lungs. This type of shock is often fatal.
Summary
The danger from electrical shock depends on
• The amount of the shocking current through the body.
• The duration of the shocking current through the body.
• The path of the shocking current through the body.
10
3
Arc Flash/ NFPA 70E
OSHA revised Subpart S to reflect updated industry practices and technology and to incorporate the 2000 edition of
NFPA 70E, Electrical Safety Requirements for Employee Workplaces, and the 2002 revision of the National Electric
Code ( NEC). NFPA 70E applies to all personnel working on energized equipment greater than 50 volts or equipment that
could produce an arc flash, which means virtually every industry has employees at risk. Under the newly revised Subpart
S— Electrical ( effective Aug. 13, 2007), OSHA as well as NCDOL has not adopted NFPA 70E in its entirety, specifically
excluding some personal protective equipment and clothing requirements in regard to arc flash.
What Is an Arc Flash?
The arc flash is the resulting discharge of energy caused by an arcing fault. An arcing fault is the unintended flow of
current through a medium not intended to carry the current. That just means that the electricity is flowing through some-thing
it should not be; in most cases that result in injury, the medium was the air. The air becomes like a piece of copper,
conducting the electricity; only with the air, you can see the massive discharge of the electrons from the discharging ele-ment.
This is the arc flash. It is lightning on a smaller, yet still deadly, scale.
What causes an arcing fault? The most common causes of an arcing fault are equipment failure, human error ( improper
placement of tools or improper use of equipment), or the conduction of electricity due to foreign particles in the air ( usu-ally
metal shavings). 4
Wearing personal protective equipment is necessary in reducing injury from electrical arc flash accidents, but it is no
substitute for proper safety training, among other best practices in arc safety.
Every day, electrical arc flash accidents injure or kill, but wearing proper personal protective equipment ( PPE) mini-mizes
accident frequency and severity. PPE alone, however, is no substitute for thorough safety training, consistently
following lockout/ tagout procedures, keeping electrical equipment well- maintained, and applying engineering controls.
Burns are not the only risk. A high- amperage arc produces an explosive pressure wave blast that can cause severe fall-related
injuries.
Four- step hazard calculations: First, establish the job’s hazard risk category. Second, determine what clothing and
equipment the hazard risk category requires. Third, identify what arc thermal performance value ( ATPV) rating is neces-sary.
Finally, select personal protective equipment that meets or exceeds the designated ATPV rating.
Arc Flash Clothing
Arc flash clothes are critically important to keep workers safe. Statistics show that five to ten times a day, a worker in
the United States is injured or killed due to an arc flashing accident. 5 The casualties resulting from these accidents are
almost always devastating to the worker involved and to the worker’s family. 5 Perhaps if these workers had been wearing
appropriately rated arc flashing protective equipment, the number of injuries and deaths could have been decreased.
Need for Protective Clothing
What steps can be taken to reduce the risk? NFPA 70E, Standard for Electrical Safety Requirements for Employee
Workplaces, sets standards and regulations for workers working around energized equipment. NFPA 70E defines neces-sary
steps to be taken to properly prevent serious injury in the event of an arc flash accident. NFPA 70E interprets that
workers within the flash protection boundary ( the area where discharged energy is greater than 1.2 cal/ cm2) must be
qualified and wearing thermally resistant and arc flash protective clothing.
11
____________
4 http:// www. lg. com/ about/ newsletter/ June04/ ArcFlash. html
5 http:// www. carolinaseca. org/ pdf/ arcflash. pdf
Arc Flash Clothing Selection
Picking the right type of arc flash protective clothing is easy. First, consult NFPA 70E, 2004 Edition, Table
130.7( C)( 9)( a), to determine to which category of risk a particular activity belongs. Second, consult Table 130.7( C)( 10) to
determine what type of clothing/ equipment is required based on the category of risk determined. Third, consult Table
130.7( C)( 11) to determine the ATPV ( arc thermal performance value) rating needed. Once you have done all this, just go
out and find the protective gear that meets or exceeds this rating. 6 One thing to remember when picking the protective
work wear is to try and ensure that no skin is exposed. Ensure that the pant legs ( if not connected to boots) completely go
down to the boot. Also ensure that the sleeves of the protective work wear go down to the hand, leaving none of the arm
exposed. And lastly, remember that the head is the most vulnerable part of the body. Do not forget to complete the arc
flash protective clothing with suitable head gear of the same ATPV rating as the rest of the work- wear plus high voltage
gloves.
NFPA 70E Table 130.7( C)( 11)
Protective Clothing Characteristics6
When nothing can be done about working within a flash protection boundary, proper arc flash protective clothing needs
to be worn. Workers need to remember that arc flash accidents do not only occur with equipment at high voltage. The
majority of arc flash accidents occur with low ( 120V) and medium voltage ( 480V) equipment. Workers who wear the
proper arc flash protective clothing will significantly reduce the risk of injury or death should an arc flash accident occur.
Summary
Always perform a flash hazard analysis and acquire the appropriate flash retardant clothing ( FRC). Care and
laundering of FRC should be taken cautiously. Employer/ employees must follow safe work practices. Employees must be
adequately trained on electrical safety, with first aid/ CPR training as needed. Whenever possible, lock out all equipment!
12
Hazard Risk Work- wear Description ATPV
Category ( 1/ 2/ 3/ 4) refers to the number of clothing layers Rating cal/ cm2
0 Untreated Cotton ( 1) n/ a
1 FR Shirt and FR Pant ( 1) 5
2 Cotton Undergarments + FR Shirt/ Pant ( 2) 8
3 Cotton Undergarments + FR Shirt/ Pant + FR Coveralls ( 3) 25
4 Cotton Undergarments + FR Shirt/ Pant + Double Layer Switching Coat
and Pant ( 4) 40
* Recommendation is 100 percent cotton.
____________
6 http:// www. labsafety. com/ refinfo/ printpage. htm? page=/ refinfo/ ezfacts/ ezf263. htm
4
Branch Circuit Wiring
Definitions
Discussion of wiring methods must be preceded by an understanding of terms used to define each specific conductor in
a typical 120/ 240 volt AC system. Refer to Figure 4 for an example of most of the following definitions. The National
Electrical Code ( NEC) is used as the reference source.
Ampacity. The current ( in amps) that a conductor can carry continuously under the conditions of use without exceed-ing
its temperature rating. When you find attachment plugs, cords or receptacle face plates that are hot to touch, this may
be an indication that too much of a load ( in amps) is being placed on that branch circuit. If the insulation on the conduc-tors
gets too hot, it can melt and cause arcing, which could start a fire.
Attachment Plug. Describes the device ( plug) that when inserted into the receptacle establishes the electrical connec-tion
between the appliance and branch circuit.
Branch Circuit. The electrical conductors between the final overcurrent device ( the service entrance panel ( SEP) in
Figure 4) protecting the circuit and the receptacle. The wiring from the SEP to the pole mounted transformer is called the
“ service.”
Circuit Breaker. Opens and closes a circuit by nonautomatic means as well as being designed to open automatically at
a predetermined current without causing damage to itself. Be alert to hot spots in circuit breaker panels indicating that the
circuit breaker is being overloaded or that there may be loose connections.
Equipment. A general term for material, fittings, devices, appliances, fixtures, apparatus and the like used as a part of,
or in connection with, an electrical installation. In Figure 4, the SEP and any associated conduit and junction boxes would
be considered equipment.
Feeder. The term given to the circuit conductors between the SEP and the final branch circuit overcurrent device. In
Figure 4 there is no feeder since the SEP is also the final branch circuit overcurrent device.
Ground. A conducting connection ( whether intentional or accidental) between an electrical circuit or equipment and
the earth, or to some conducting body that serves in place of the earth. It is important to remember that a conducting body
can be in the ceiling and that we must not
think of ground as restricted to earth. This
is why maintenance personnel may not
realize that a ground loop exists in the
space above a drop ceiling, due to the elec-trical
conduit and other grounded equip-ment
in that space.
Grounded Conductor. The conductor
in the branch circuit wiring that is inten-tionally
grounded in the SEP. This conduc-tor
is illustrated in Figure 4. From the SEP
to the transformer the same electrical path
is referred to as the neutral. From the final
overcurrent device to the receptacle the
conductor is referred to as the grounded
conductor.
13
120 Volts
120 Volts
Neutral
Grounding Electrode
Conductor on
Premises
Equipment- grounding
Conductor ( green or bare)
Utility
Supply
Service
Ground
Circuit Breaker
Grounded
Conductor
( white or gray)
“ Hot” Conductor
( black or red)
Typical Pole
Transformer
Service
Entrance
Panel
Nickel or
Light- colored
Terminal
Green
Hexagonal- head
Terminal Screw
Brass colored
Terminal
Primary Lines
2.4– 13 kV
Figure 4
Branch Circuit Wiring
Grounding Conductor, Equipment. The conductor used to connect the noncurrent- carrying metal parts of equipment,
raceways and other enclosures to the system grounded conductor at the SEP. The equipment grounding conductor path is
allowed to be a separate conductor ( insulated or noninsulated), or where metal conduit is used, the conduit can be used as
the conductor. There are some exceptions to this such as in hospital operating and intensive care rooms. The equipment
grounding conductor is the human safety conductor of the electrical system in that it bonds all noncurrent- carrying metal
surfaces together and then connects them to ground. By doing this we can prevent a voltage potential difference between
the metal cabinets and enclosures of equipment and machinery. This conductor also acts as a low impedance path ( in the
event of a voltage fault to the equipment case or housing) so that if high fault current is developed, the circuit breaker or
fuse will be activated quickly.
Grounding Electrode Conductor. Used to connect the grounding electrode to the equipment grounding conductor
and/ or to the grounded conductor of the circuit at the service equipment or at the source of a
separately derived system ( see Figure 4).
Overcurrent. Any current in excess of the rated current of equipment or the ampacity of a conductor is considered
overcurrent. This condition may result from an overload, short circuit or a ground fault.
Wiring Methods
The NEC requires the design and installation of electrical wiring to be consistent throughout the facility. To accomplish
this, it is necessary to follow NEC requirements. For 120 volt grounding- type receptacles, the following wiring connec-tions
are required ( see Figure 4).
• The ungrounded or “ hot” conductor ( usually with black or red insulation) is connected to the brass colored terminal
screw. This terminal and the metal tension springs form the small slot receiver for any appliance attachment plug.
An easy way to remember the color coding is to remember “ black to brass” or the initials “ B & B.”
• The “ grounded conductor” insulation is generally colored white ( or gray) and should be fastened to the silver or light
colored terminal. This terminal and the metal tension springs form the large slot for a polarized attachment plug. An
easy way to remember this connection is to think “ white to light.”
• The equipment grounding conductor path can be a conductor, or where metal conduit is used, the conduit can be sub-stituted
for the conductor. If the latter is used, you must monitor the condition of the conduit system to ensure that it
is not damaged or broken. Any “ open” in the conduit system will eliminate the equipment grounding conductor
path. Additionally, the condition of the receptacles must be monitored to ensure that they are securely fastened to
the receptacle boxes. When a third wire is run to the receptacle either in a conduit or as a part of a nonmetallic
sheathed cable assembly, the conductor must be connected to the green colored terminal on the receptacle.
These wiring methods must be used to ensure that the facility is correctly wired. Circuit testing methods will be dis-cussed
in Part 5. In older homes, knob and tube or other two- wire systems may be present. The NEC requires that ground-ing
type receptacles be used as replacements for existing nongrounding types and be connected to a grounding conductor.
An exception is that where a grounding means does not exist in the enclosure, either a nongrounding or a GFCI- type
receptacle must be used. A grounding conductor must not be connected from the GFCI receptacle to any outlet supplied
from the GFCI receptacle. The exception further allows nongrounding type receptacles to be replaced with the grounding
type where supplied through a GFCI receptacle.
Plug and Receptacle Configurations
Attachment plugs are devices that are fastened to the end of a cord so that electrical contact can be made between the
conductor in the equipment cord and the conductors in the receptacle. The plugs and receptacles are designed for different
voltages and currents, so that only matching plugs will fit into the correct receptacle. In this way, a piece of equipment
rated for one voltage and/ or current combination cannot be plugged into a power system that is of a different voltage or
current capacity.
The polarized three- prong plug is designed with the equipment grounding prong slightly longer than the two parallel
blades. This provides equipment grounding before the equipment is energized. Conversely, when the plug is removed
from the receptacle, the equipment grounding prong is the last to leave, ensuring a grounded case until power is removed.
The parallel line blades maybe the same width on some appliances since the three- prong plug can only be inserted in one
14
way. A serious problem results whenever a person breaks or cuts off the grounding prong. This not only voids the safety
of the equipment grounding conductor but allows the attachment plug to be plugged in with the correct polarity or with
the wrong polarity.
Figure 5 illustrates some of the
National Electrical Manufacturers
Association ( NEMA) standard plug and
receptacle connector blade configura-tions.
Each configuration has been devel-oped
to standardize the use of plugs and
receptacles for different voltages,
amperes, and phases from 115 through
600 volts and from 10 through 60 amps,
and for single- and three- phase systems.
You should be alert to jury- rigged
adaptors used to match, as an example, a
50 amp attachment plug to a 20 amp
receptacle configuration. Using these
adaptors poses the danger of mixing volt-age
and current ratings and causing fire
and/ or shock hazards to personnel using
equipment. Equipment attachment plugs
and receptacles should match in voltage
and current ratings to provide safe power
to meet the equipment ratings. Also the
attachment plug cord clamps must be
secured to the cord to prevent any strain
or tension from being transmitted to the ter-minals
and connections inside the plug.
Understanding Reverse Polarity
The NEC recognizes the problem of reverse polarity. It states that no grounded conductor may be attached to any ter-minal
or lead so as to reverse the designated polarity. Many individuals experienced with electrical wiring and appliances
think that reverse polarity is not hazardous. A few example situations should heighten your awareness of the potential
shock hazard from reverse polarity.
An example of one hazardous situation
would be an electric hand lamp. Figure 6
illustrates a hand lamp improperly wired and
powered.
When the switch is turned off, the shell of
the lamp socket is energized. If a person acci-dentally
touched the shell ( while changing a
bulb) with one hand and encountered a
ground loop back to the transformer, a shock
could result. If the lamp had no switch and
was plugged in as shown, the lamp shell
would be energized when the plug was insert-ed
into the receptacle. Many two- prong plugs
have blades that are the same size, and the
right or wrong polarity is just a matter of
chance. If the plug is reversed ( Figure 7), the
voltage is applied to the bulb center terminal
15
Only
5- 15R 5- 15P
5- 20R 5- 20P
10- 50R 10- 50P
10- 30R 10- 30P
6- 30R 6- 30P
125/ 250- Volt, 30- Ampere
Receptacle and Plug
250- Volt, 30- Ampere
Receptacle and Plug
125/ 250- Volt, 50- Ampere
Receptacle and Plug
20- Ampere Plug,
125- Volt Receptacles
and Plugs
Either
15- Ampere Plug
15 Ampere
20 Ampere
W
X Y
W
Y X
W
Y X
W
X Y
W
G
W
G
W
G
W
G
G G
Figure 5
Plug and Receptacle Configurations
Switch
OFF Position
Bulb
Metal Guard
Ground Potential
Equipment Plug
Receptacle
“ Hot”
“ Hot”
SEP
120 Volt
Transformer
Figure 6
Hand Lamp— Reverse Polarity
and the shell is at ground potential. Contact
with the shell and ground would not create a
shock hazard in this situation. In this example
the hand lamp is wired correctly.
Another example is provided by electric
hair dryers or other plastic- covered electrical
appliances. Figure 8 illustrates a hair dryer
properly plugged into a receptacle with the
correct polarity.
You will notice that the switch is single
pole- single throw ( SPST). When the appli-ance
is plugged in with the switch in the hot
or 120 volt leg, the voltage stops at the switch
when it is in the off position. If the hair dryer
were accidentally dropped into water, current could flow out of the plastic housing, using the water as the conducting
medium. The water does not short out the appliance since the exposed surface area of the hot wire connection to the
switch terminal offers such a high resis-tance.
This limits the current flow to less
than 1 mA ( correct polarity). The fault
current is not sufficient to trip a 20- amp
circuit breaker. To trip the circuit break-er,
there would have to be a line- to- line
short that would cause an excess of 20
amps ( 20,000 mA). Should a person try
to retrieve the appliance from the water
while it is still plugged into the outlet?
In this configuration, the fault current
would be extremely low ( unless the
switch were in the on position). Since
you have no way to tell if the polarity is
correct, don’t take chances. NEVER
REACH INTO WATER TO RETRIEVE
AN APPLIANCE. Always unplug the appliance first, then retrieve the appliance and dry it out.
If the appliance is plugged in as shown in Figure 9, when the switch is off, voltage will be present throughout all the
internal wiring of the appliance. Now if it is dropped into water, the drastically increased “ live” surface area will allow a
drastic increase in the available electric current ( I = E/ R). A person who accidentally tries to retrieve the dryer would be in
a hazardous position because the voltage in the water could cause current to flow through the body ( if another part of the
body contacts a ground loop). This illus-trates
the concept that reverse polarity is a
problem whenever appliances are used
with plastic housings in areas near sinks,
or where the appliance is exposed to rain
or water. Remember, most motorized
appliances have air passages for cooling.
Wherever air can go, so can moisture and
water.
If the appliance had a double pole- dou-ble
throw switch ( DPDT), it would make
no difference how the plug was positioned
in the outlet. The hazard would be mini-mized
since the energized contact surface
would be extremely small. If the appliance
16
Switch
OFF Position
Bulb
Metal Guard
Ground Potential
Equipment Plug
Receptacle
“ Hot”
“ Hot”
“ Hot”
SEP
120 Volt
Transformer
Figure 7
Hand Lamp— Correct Polarity
Service
Entry
Panel
( SEP)
120 Volt
Transformer
Plug
Receptacle
“ Hot”
“ Hot”
Hair Dryer
Switch
Figure 8
Hair Dryer— Correct Polarity
Service
Entry
Panel
( SEP)
120 Volt
Transformer
Plug
Receptacle
“ Hot”
“ Hot”
Hair Dryer
Switch
Figure 9
Hair Dryer— Reverse Polarity
were dropped into water, a high resistance contact in the water and a resulting low available fault current ( I = E/ R) would
result. Later, we will see how GFCIs can be used to protect against shock hazards when using appliances with noncon-ductive
housings around water.
Remember that any electric appliance dropped in water or accidentally exposed to moisture should be considered as ener-gized.
The electric power must be safely removed from the appliance before it is retrieved or picked up. You never know if
the appliance is plugged in with the right polarity without test equipment. Do not take chances. Remove the power first.
Grounding Concepts
Grounding falls into two safety categories. It is important to distinguish between “ system grounding” and “ equipment
grounding.” Figure 10 illustrates these two grounding components. The difference between these two terms is that system
grounding actually connects one of the current carrying conductors from the supply transformer to ground. Equipment
grounding connects or bonds all of the noncurrent- carrying metal surfaces together and then is connected to ground.
System grounding ( Figure 10) at the transformer provides a grounding point for the power company surge and light-ning
protection devices. In conjunction with the system grounding at the SEP, the voltage across system components is
limited to a safe value should they be subject-ed
to lightning or high voltage surges. The
system grounding at the SEP also helps to
limit high voltages from entering the electri-cal
system beyond the SEP. It is important to
check all of the connections both indoors and
outdoors since many times they are exposed
to moisture, chemicals and physical damage.
Equipment grounding does two things.
First, it bonds all noncurrent- carrying metal
surfaces together so that there will be no
potential difference between them. Second, it
provides a path for current to flow under
ground fault conditions. The equipment
grounding path must have low impedance to
ensure rapid operation of the circuit overcur-rent
device should a “ hot” to ground fault
occur.
Figure 11 depicts some common equip-ment
faults that can occur. A problem with
the equipment grounding system is that under
normal conditions it is not a current- carrying
conductor and a fault would not be readily
detected. Necessary visual inspection will not
provide an operational verification. In Figure
11 we can test the condition of the equipment
grounding conductor using test procedures in
this guide. To test the quality of the branch
circuit equipment grounding system, a special
tester called a ground loop impedance tester
is needed. It is generally recommended that
an impedance of 0.5 ohms be achieved in the
equipment grounding conductor path. In case
of a “ hot” to ground fault, the fault current
would quickly rise to a value ( I = E/ R =
120/ 0.5 = 240 amps) necessary to trip a 20
amp fuse or breaker.
17
Equipment Grounding
Equipment
Grounding
Conductor
Electrical Symbol
For Ground
Service
Entrance
Grounded
Conductor or
Neutral
Transformer
Secondary
Primary
Most Metallic Raceways, Cable Sheaths
and Cable Armor That Are Continuous
and Utilize Proper Fittings May Serve as
the Equipment Grounding Conductor. A
Separate Grounding Conductor is Needed
When Plastic Conduit, Non- metallic
Sheathed Cable or Other Wiring Methods
Are Used That Are Not Approved as
Grounding Methods.
Figure 10
System and Equipment Grounding
? ? Fault
White
Black
Receptacle Ground
Fault
Tool or
Appliance
Grounding Prong
Missing
Ground Fault
Current
Wire or
Conduit
Service
Entrance
Source
Figure 11
Equipment Grounding Faults
If there is an open or break in the conduit system, as shown in Figure 11, a fault in a power tool with a good grounding
prong would allow the voltage to be placed directly on the ungrounded conduit. This would create a serious shock hazard
to anyone touching the conduit with one hand while touching a ground loop with the other. The missing ground prong
( with a good conduit or equipment grounding path) would create a serious shock hazard to the person if the ground loop
through the feet was low resistance ( e. g., wet earth or concrete and wet shoes). It is important to emphasize the need for
low impedance on the equipment grounding loop. If the ground fault current loop in Figure 11 were 25 ohms and the
body resistance of the person were 850 ohms, then the 25 ohm ground loop resistance would be too high to cause enough
circuit breaker current to trip it open. In this case, the person would then receive multiples of current considered deadly
( 141 mA in this case) through the body, causing death in most instances. Remember that standard circuit breakers are for
equipment and fire protection, not people protection. However, ground fault circuit interrupter ( GFCI) circuit breakers are
specifically designed for people protection.
Burns Caused by Electricity
The most common shock- related, nonfatal injury is a burn. Burns caused by electricity may be of three types: electrical
burns, arc burns and thermal contact burns. Electrical burns can result when a person touches electrical wiring or
equipment that is used or maintained improperly. Typically such burns occur on the hands. Electrical burns are one of the
most serious injuries you can receive. They need to be given immediate attention. Additionally, clothing may catch fire and
a thermal burn may result from the heat of the fire.
• Electrical shocks cause burns.
Arc- blasts occur when powerful, high- amperage currents arc through the air. Arcing is the luminous electrical dis-charge
that occurs when high voltages exist across a gap between conductors and current travels through the air. This situ-ation
is often caused by equipment failure due to abuse or fatigue. Temperatures as high as 35,000 F have been reached in
arc- blasts.
• arc- blast— explosive release of molten material from equipment caused by high- amperage arcs
• arcing— the luminous electrical discharge ( bright, electrical sparking) through the air that occurs when high volt-ages
exist across a gap between conductors
There are three primary hazards associated with an arc- blast.
1. Arcing gives off thermal radiation ( heat) and intense light, which can cause burns. Several factors affect the degree
of injury, including skin color, area of skin exposed and type of clothing worn. Proper clothing, work distances and
overcurrent protection can reduce the risk of such a burn.
2. A high- voltage arc can produce a considerable pressure wave blast. A person 2 feet away from a 25,000- amp arc
feels a force of about 480 pounds on the front of the body. In addition, such an explosion can cause serious ear
damage and memory loss due to concussion. Sometimes the pressure wave throws the victim away from the arc-blast.
While this may reduce further exposure to the thermal energy, serious physical injury may result. The pres-sure
wave can propel large objects over great distances. In some cases, the pressure wave has enough force to snap
off the heads of steel bolts and knock over walls.
3. A high- voltage arc can also cause many of the copper and aluminum components in electrical equipment to melt.
These droplets of molten metal can be blasted great distances by the pressure wave. Although these droplets harden
rapidly, they can still be hot enough to cause serious burns or cause ordinary clothing to catch fire, even if you are
10 feet or more away.
18
Electrical Fires
Electricity is one of the most common causes of fires and thermal burns in homes and workplaces. Defective or mis-used
electrical equipment is a major cause of electrical fires. If there is a small electrical fire, be sure to use only a Class
C or multi- purpose ( ABC) fire extinguisher, or you might make the problem worse. All fire extinguishers are marked with
letter( s) that tell you the kinds of fires they can put out. Some extinguishers contain symbols, too. The letters and symbols
are explained below ( including suggestions on how to remember them).
A( think: Ashes) = paper, wood, etc.
B( think: Barrel) = flammable liquids
C( think: Circuits) = electrical fires
Here are a couple of fire extinguishers at a worksite. Can you tell what types of
fires they will put out?
19
Five technicians were performing preventive maintenance on the electrical system of a railroad maintenance facility.
One of the technicians was assigned to clean the lower compartment of an electrical cabinet using cleaning fluid in
an aerosol can. He began to clean the upper compartment as well. The upper compartment was filled with live cir-cuitry.
When the cleaning spray contacted the live circuitry, a conductive path for the current was created. The cur-rent
passed through the stream of fluid, into the technician’s arm and across his chest. The current caused a loud
explosion. Co- workers found the victim with his clothes on fire. One worker put out the fire with an extinguisher, and
another pulled the victim away from the compartment with a plastic vacuum cleaner hose. The paramedics
responded in 5 minutes. Although the victim survived the shock, he died 24 hours later of burns.
This death could have been prevented if the following precautions had been taken:
• Before doing any electrical work, de- energize all circuits and equipment, perform lockout/ tagout, and test circuits
and equipment to make sure they are de- energized.
• The company should have trained the workers to perform their jobs safely.
• Proper personal protective equipment ( PPE) should always be used.
• Never use aerosol spray cans around high- voltage equipment.
This extinguisher can only be used on Class B
and Class C fires.
This extinguisher can only be used on
Class A and Class C fires.
Thermal burns may result if an explosion occurs when electricity ignites an explosive mixture of material in the air.
This ignition can result from the buildup of combustible vapors, gases or dusts. OSHA standards, the NEC and other safe-ty
standards give precise safety requirements for the operation of electrical systems and equipment in such dangerous
areas. Ignition can also be caused by overheated conductors or equipment, or by normal arcing at switch contacts or in
circuit breakers.
Summary
Burns are the most common injury caused by electricity. The three types of burns are electrical burns, arc burns and
thermal contact burns.
20
Note: However, do not try to put out fires unless you have
received proper training. If you are not trained, the best thing
you can do is evacuate the area and call for help.
5
Branch Circuit and Equipment Testing
Testing Branch Circuit Wiring
Branch circuit receptacles should be tested periodically. The frequency of testing should be established on the basis of
outlet usage. In shop areas, quarterly testing may be necessary. Office areas may only need annual testing. A preventive
maintenance program should be established. It is not unusual to find outlets as old as the facility. For some reason, a pop-ular
belief exists that outlets never wear out. This is false. For example, outlets take severe abuse from employees discon-necting
the plug from the outlet by yanking on the cord. This can put severe strain on the contacts inside the outlet as well
as on the plastic face.
The electrical receptacle is a critical electrical system component. It must provide a secure mechanical connection for
the appliance plug so that there is a continuous electrical circuit for each of the prongs. Receptacles must be wired correct-ly,
or serious injuries can result from their use. For this reason, the following two- step testing procedure is recommended.
Receptacle Testing
Step 1
Plug in a three- prong receptacle circuit tester and note the combination of the
indicator lights ( see Figure 12). The tester checks the receptacle for the proper con-nection
of the grounding conductor, wiring polarity and other combinations of
wiring errors. If the tester checks the receptacle as OK, proceed to the next step. If
the tester indicates a wiring problem, have it corrected as soon as possible. Retest
after the problem is corrected.
Step 2
After the outlet has been found to be
wired ( electrically) correct, the receptacle
contact tension test must be made. A typical tension tester is shown in Figure 13.
The tension should be 8 ounces or more. If it is less than 8 ounces, have it
replaced. The first receptacle function that loses its contact tension is usually the
grounding contact circuit. The plug grounding prong is the last part to leave the
receptacle. Because of the leverage ( due to its length) it wears out the tension of
the receptacle contacts quicker than the parallel blade contacts. Since this is the
human safety portion of the grounding system, it is very important that this contact
tension be proper. In determining the frequency of testing, the interval must be
based on receptacle usage. All electrical maintenance personnel should be
equipped with receptacle circuit and tension testers. Other maintenance employees could also be equipped with testers
and taught how to use them. In this manner, the receptacles can be tested before maintenance workers use them. Home
receptacles should also be tested.
Testing Extension Cords
New extension cords should be tested before being put into service. Many inspectors have found that new extension
cords have open ground or reverse polarity. Do not assume that a new extension cord is correct. Test it.
Extension cord testing and maintenance are extremely important. The extension cord takes the electrical energy from a
fixed outlet or source and provides this energy at a remote location. The extension cord must be wired correctly, or it can
become the critical fault path.
Testing of extension cords new and used should use the same two steps as used in electrical outlet testing. These two
steps are:
21
Figure 12
Receptacle Tester
Figure 13
Receptacle Tension Tester
Step 1
Plug the extension cord into an electrical outlet that has successfully passed the outlet testing procedure. Plug in any
three- prong receptacle circuit tester into the extension receptacle and note the combination of indicator lights. If the tester
checks the extension cord as OK, proceed to the next step. If the tester indicates a faulty condition, repair or replace the
cord. Once the extension cord is correctly repaired and passes the three- prong circuit tester test, proceed to step 2.
Step 2
Plug a reliable tension tester into the receptacle end. The parallel receptacle contact tension and the grounding contact
tension should check out at 8 ounces or more. If the tension is less than 8 ounces, the receptacle end of the extension cord
must be repaired. As with fixed electrical outlets, the receptacle end of an extension cord loses its grounding contact ten-sion
first. This path is the critical human protection path and must be both electrically and mechanically in good condition.
Testing Plug- and Cord- Connected Equipment
The last element of the systems test is the testing of plug- and cord- connected equipment. The electrical inspector
should be on the alert for jury- rigged repairs made on power tools and appliances. Many times a visual inspection will
disclose three- prong plugs with the grounding prong broken or cut off. In such instances, the grounding path to the equip-ment
case has been destroyed. ( The plug can now also be plugged into the outlet in the reverse polarity configuration.)
Double insulated equipment generally has a nonconductive case and will not be tested using the procedures discussed
below. Some manufacturers that have listed double insulation ratings may also provide the three- prong plug to ground
any exposed noncurrent- carrying metal parts. In these cases, the grounding path continuity can be tested.
A common error from a maintenance standpoint is the installation of a three- prong plug on a two- conductor cord to the
appliance. Obviously, there will be no grounding path if there are only two conductors in the cord. Some hospital grade
plugs have transparent bases that allow visual inspection of the electrical connection to each prong. Even in that situation,
you should still perform an electrical continuity test.
Maintenance shops may have commercial power tool testing equipment. Many power tool testers require the availabili-ty
of electric power. The ohmmeter can be used in the field and in locations where electric power is not available or is not
easily obtained. Plug- and cord- connected equipment tests are made on de- energized equipment. Testing of de- energized
equipment in wet and damp locations can also be done safely.
The plug- and cord- connected equipment test using a self- contained battery- powered ohmmeter is simple and straight-forward.
The two- step testing sequence that can be performed on three- prong plug grounded equipment follows:
Step 1— Continuity Test— Ground Pin to Case Test
Set the ohmmeter selector switch to the lowest scale
( such as R X 1). Zero the meter by touching the two test
probes together and adjusting the meter indicator to zero.
Place one test lead ( tester probe) on the grounding pin of
the de- energized equipment as shown in Figure 14. While
holding that test lead steady, take the other test lead and
make contact with an unpainted surface on the metal case
of the appliance. You should get a reading of less than 1
ohm. If the grounding path is open, the meter will indicate
infinity. If there is no continuity, then the appliance must
be tagged out and removed from service. If the appliance
grounding path is OK, proceed to step 2.
22
White
MOTOR
Green
Silver
Brass
Black
Metal Case
Tester Probe
Tester Probe
SPST
Switch
R X 1
Scale
Reading 1 ohm
or less
Figure 14
Continuity Test
Step 2— Leakage Test— Appliance Leakage Test
Place the ohmmeter selector switch on the highest ohm test position ( such as R X 1,000). Set the meter at zero. Place
one test lead on an unpainted surface of the appliance case ( see Figure 15), then place the other test lead on one of the
plug’s parallel blades.
Observe the reading. The ideal is close to infinity. ( If
a reading of less than 1 meg- ohm is noted, return the
appliance to maintenance for further testing.) With one
test lead still on the case, place the other test lead on
the remaining parallel blade and note the ohmmeter
reading. A reading approaching infinity is required.
( Again, anything less than 1 meg- ohm should be
checked by a maintenance shop.)
23
White
MOTOR
Green
Silver
1.
2.
Brass
Black
Metal Case
Tester Probe
Tester Probe
SPST
Switch
R X 1,000
Scale
Reading
8
Figure 15
Leakage Test
6
Voltage Detector Testing
Operation
When you are conducting an electrical inspection, a voltage detector should be used in conjunction with the circuit
tester, ohmmeter and tension tester. Several types of these devices are inexpensive and commercially available.
These testers are battery powered and are constructed of nonconduct-ing
plastic. Lightweight and self- contained, these testers make an ideal
inspection tool.
The voltage detector works like a radio receiver in that it can receive
or detect the 60 hertz electromagnetic signal from the voltage waveform
surrounding an ungrounded (“ hot”) conductor. Figure 17 illustrates the
detector being used to detect the “ hot” conductor in a cord connected to
a portable hand lamp. When the front of the detector is placed near an
energized “ hot” or ungrounded conductor, the tester will provide an
audible as well as a visual warning. In Figure 17 if the plug were dis-connected
from the receptacle, the detector would not sound an alarm
since there would not be any voltage waveform present.
Typical Voltage Detector Uses
The detector can also be used to test for properly
grounded equipment. When the tester is positioned on a
properly grounded power tool ( e. g., the electric drill in
Figure 18), the tester will not sound a warning. The rea-son
is that the electromagnetic field is shielded from the
detector so that no signal is picked up. If the grounding
prong had been removed and the drill were not ground-ed,
the electromagnetic waveform would radiate from
the drill and the detector would receive the signal and
give off an alarm.
As illustrated in Figure 18, the detector can be used
to test for many things during an inspection.
Receptacles can be checked for proper AC polarity.
Circuit breakers can be checked to determine if they are
on or off. All fixed equipment can be checked for prop-er
grounding. If the detector gives a warning indication
on any equipment enclosed by metal, perform further
testing with a volt- ohmmeter. Ungrounded equipment
can be ungrounded or, in addition, there may be a fault
to the enclosure making it “ hot” with respect to ground.
An experienced inspector should use the voltmeter to
determine if there is voltage on the enclosure. The volt-age
detector should be used as an indicating tester and
qualitative testing should be accomplished with other
testing devices such as a volt- ohmmeter. The use of the
detector can speed up the inspection process by allow-ing
you to check equipment grounding quickly and
safely.
24
Switch
Audible Alarm
Test Marker
“ Red” Warning Light
“ Green” Power “ On” Light
Figure 16
Voltage Detector
SEP
“ Hot”
“ Hot”
120 Volt
Receptacle
Transformer
Figure 17
Testing for “ Hot” Conductor
Check Fixed
Equipment
Grounding
Easily Check
Power Tools for
Proper Grounding
Safely and Quickly
Check for
Voltage
at AC Outlets
Quickly
Check
Circuit
Breakers
Figure 18
Typical Voltage Detector Uses
Voltage Versus Detection Distance
Another unique feature of the voltage detector shown in Figure 18 is that it is voltage sensitive. Figure 19 lists dis-tances
versus voltage at which the detector “ red” light will turn on. As an example, a conductor energized with 120 volts
AC can be expected to be detected from 0 to 1 inch from the conductor.
The transformer secondary wiring on a furnace
automatic ignition system rated at 10,000 volts
can be expected to be detected from 6 to 7 feet
away. You can use this feature to assist in making
judgments regarding the degree of hazard and
urgency for obtaining corrective action.
When you are using the voltage detector, you
must understand how it operates to interpret its
warning light properly. If you apply the tester to
an energized power tool listed as double insulat-ed,
you will see the red warning light turn on. In
this situation, the voltage waveform is detected
because there is no metal enclosure to shield the
waveform. The same would occur if you were to
test in any of the typewriters used in offices. This does not mean that the plastic or nonmetallic enclosed equipment is
unsafe, only that it is energized and not a grounding type piece of equipment. The examples and explanation of the opera-tional
features should be understood fully. Using the detector provides the opportunity for finding many electrical hazards
that others may have overlooked.
25
To Determine Approximate Voltage:
Slowly approach the circuitry being tested with the front sensor of the
unit and observe the distance at which the red LED light glows along
with the “ beep” sound. Use the chart below to determine the approxi-mate
voltage in the circuitry.
VOLTAGE ( V) 100 200 600 1K 5K 9K
DISTANCE ( inch) 0– 1 1– 2 3– 5 15 5 ft 6– 7 ft up
These figures may vary due to conditions governing the testing, i. e.,
static created by your standing on grounded material, carpets, etc.
Figure 19
Voltage vs. Detection Distance
7
Ground Fault Circuit Interrupters
Operational Theory
The ground fault circuit interrupter ( GFCI) is a fast- acting device that monitors the current flow to a protected load. The
GFCI can sense any leakage of current when current returns to the supply transformer by any electrical loop other than
through the white ( grounded conductor) and the black ( hot) conductors. When any “ leakage current” of 5 mA or more is
sensed, the GFCI, in a fraction of a second, shuts off the current on both the “ hot” and grounded conductors, thereby interrupt-ing
the fault current to the appliance and the fault loop.
This is illustrated in Figure 20. As long as I1 is equal to I2 ( normal appliance operation with no ground fault leakage),
the GFCI switching system remains closed. If a fault occurs between the metal case of an appliance and the “ hot” conduc-tor,
fault current I3 will cause an imbalance ( 5 mA or greater for human protection) allowing the GFCI switching system
to open ( as illustrated) and the removal of power from both the white and the “ hot” conductors.
Another type of ground fault can occur
when a person comes in contact with a “ hot”
conductor directly or touches an appliance
with no ( or a faulty) equipment grounding
conductor. In this case I4 represents the fault
current loop back to the transformer. This
type of ground fault is generally the type that
individuals are exposed to.
The GFCI is intended to protect people. It
de- energizes a circuit, or portion thereof, in
approximately 1/ 40 of a second when the
ground fault current exceeds 5 mA. The
GFCI should not be confused with ground
fault protection ( GFP) devices that protect
equipment from damaging line- to- ground
fault currents. Protection provided by GFCIs
is independent of the condition of the equip-ment
grounding conductor. The GFCI can protect personnel even when the equipment grounding conductor is accidental-ly
damaged and rendered inoperative.
The NEC requires that grounding type receptacles be used as replacements for existing nongrounding types. Where a
grounding means does not exist in the receptacle enclosure, the NEC allows either a nongrounding or GFCI receptacle.
Nongrounding type receptacles are permitted to be replaced with grounding type receptacles when powered through a
GFCI.
Remember that a fuse or circuit breaker cannot provide “ hot” to ground loop protection at the 5 mA level. The fuse or
circuit breaker is designed to trip or open the circuit if a line to line or line to ground fault occurs that exceeds the circuit
protection device rating. For a 15 amp circuit breaker, a short in excess of 15 amps or 15,000 mA would be required. The
GFCI will trip if 0.005 amps or 5 mA start to flow through a ground fault in a circuit it is protecting. This small amount
( 5 mA), flowing for the extremely short time required to trip the GFCI, will not electrocute a person but will shock the
person in the magnitude previously noted.
Typical Types of GFCIs
GFCIs are available in several different types. Figure 21 illustrates three of the types available ( portable, circuit break-er
and receptacle).
26
“ Hot”
Supply
I 1
I 2
I 3
I 4
N. C.
Protected
Load
Equipment
Grounding
Conductor
Sensing and
Test Circuit
External
Ground
Fault
Internal
Equipment
Fault
Grounded
Conductor
Differential
Transformer GFCI Protection Device
Figure 20
Circuit Diagram for a GFCI
Circuit breakers can be purchased with the GFCI protection built in. These combination types have the advantage of
being secured in the circuit breaker panel to prevent unauthorized persons from having access to the GFCI. The receptacle
type GFCI is the most convenient in that it can be tested and/ or reset at the location where it is used. It can also be
installed so that if it is closest to the circuit breaker panel, it will provide GFCI protection to all receptacles on the load
side of the GFCI. The portable type can be carried in a maintenance person’s tool box for instant use at a work location
where the available AC power is not GFCI protected. Other versions of GFCIs are available including multiple outlet
boxes for use by several power tools and for outdoor applications. A new type is available that can be fastened to a per-son’s
belt and carried to the job location.
The difference between a regular electrical recep-tacle
and a GFCI receptacle is the presence of the
TEST and RESET buttons. The user should follow
the manufacturer’s instructions regarding the testing
of the GFCI. Some instructions recommend pushing
the TEST button and resetting it monthly. Defective
units should be replaced immediately.
Be aware that a GFCI will not protect the user
from line ( hot) to line ( grounded conductor) electri-cal
contact. If a person were standing on a surface
insulated from ground ( e. g., a dry, insulated floor
mat) while holding a faulty appliance with a “ hot”
case in one hand, then reached with the other hand to
unplug an appliance plug that had an exposed
grounded ( white) conductor, a line to line contact
would occur. The GFCI would not protect the person
( since there is no ground loop). To prevent this type
of accident from occurring, it is very important to have an ensured equipment grounding conductor inspection program in
addition to a GFCI program. GFCIs do not replace an ongoing electrical equipment inspection program. GFCIs
should be considered as additional protection against the most common form of electrical shock and electrocution: the
line (“ hot”) to ground fault.
GFCI Uses
GFCIs should be used in dairies, breweries, canneries, steam plants, construction sites, and inside metal tanks and boil-ers.
They should be used where workers are exposed to humid or wet conditions and may come in contact with ground or
grounded equipment. They should be used in any work environment that is or can become wet and in other areas that are
highly grounded.
Nuisance GFCI Tripping
When GFCIs are used in construction activities, the GFCI should be located as close as possible to the electrical equip-ment
it protects. Excessive lengths of electrical temporary wiring or long extension cords can cause ground fault leakage
current to flow by capacitive and inductive coupling. The combined leakage current can exceed 5 mA causing the GFCI
to trip—“ nuisance tripping.” GFCIs are now available that can be fastened to your belt. You can plug the extension cord
into the GFCI and use the protected duplex to power lighting and power tools. This is the ideal GFCI for construction and
maintenance workers. It also allows the GFCI to be near the user location, thereby reducing nuisance trips.
Other nuisance tripping may be caused by:
• Outdoor GFCIs not protected from rain or water
• Bad electrical equipment with case to hot conductor fault
• Too many power tools on one GFCI branch
• Resistive heaters
• Coiled extension cords ( long lengths)
• Poorly installed GFCI
27
Receptacle
Type
Circuit Breaker
Type
Portable Type
Figure 21
Typical Types of GFCIs
• Electromagnetic induced current near high voltage lines
• Portable GFCI plugged into a GFCI protected branch circuit
Remember that a GFCI does not prevent shock. It limits the duration of the shock so that the heart does not go
into ventricular fibrillation. The shock lasts about 1/ 40 of a second and can be intense enough to knock a person off a
ladder or otherwise cause an accidental injury. Also, remember that the GFCI does not protect against line- to- line
shock. Be sure to check insulation and connections on power tools each time before use.
Overview of the Safety Model
What Must Be Done to Be Safe?
Use the three- stage safety model: recognize, evaluate and control hazards. To be safe, you must think about your job
and plan for hazards. To avoid injury or death, you must understand and recognize hazards. You need to evaluate the situ-ation
you are in and assess your risks. You need to control hazards by creating a safe work environment, by using safe
work practices, and by reporting hazards to a supervisor, trainer or appropriate person( s).
• Use the safety model to recognize, evaluate and control hazards.
• Identify electrical hazards.
• Don’t listen to reckless, dangerous people.
If you do not recognize, evaluate and control hazards, you may be injured or killed by the
electricity itself, electrical fires or falls. If you use the safety model to recognize, evaluate, and
control hazards, you are much safer.
Stage 1: Recognize Hazards
The first part of the safety model is recognizing the hazards around you. Only then can you
avoid or control the hazards. It is best to discuss and plan hazard recognition tasks with your
co- workers. Sometimes we take risks ourselves, but when we are responsible for others, we are
more careful. Sometimes others see hazards that we overlook. Of course, it is possible to be
talked out of our concerns by someone who is reckless or dangerous. Don’t take a chance.
Careful planning of safety procedures reduces the risk of injury. Decisions to lock out and tag out circuits and equipment
need to be made during this part of the safety model. Plans for action must be made now.
Stage 2: Evaluate Hazards
When evaluating hazards, it is best to identify all possible hazards first, then evaluate the
risk of injury from each hazard. Do not assume the risk is low until you evaluate the hazard.
It is dangerous to overlook hazards. Jobsites are especially dangerous because they are
always changing. Many people are working at different tasks. Jobsites are frequently
exposed to bad weather. A reasonable place to work on a bright, sunny day might be very
hazardous in the rain. The risks in your work environment need to be evaluated all the time.
Then, whatever hazards are present need to be controlled.
• Evaluate your risk.
Stage 3: Control Hazards
Once electrical hazards have been recognized and evaluated, they must be controlled. You control electrical hazards in
two main ways: ( 1) create a safe work environment and ( 2) use safe work practices. Controlling electrical hazards ( as
well as other hazards) reduces the risk of injury or death.
28
Report hazards to your
supervisor, trainer or
other personnel as
appropriate.
OSHA regulations, the NEC and the National Electrical Safety Code ( NESC) provide a wide range of safety informa-tion.
Although these sources may be difficult to read and understand at first, with practice they can become very useful
tools to help you recognize unsafe conditions and practices. Knowledge of OSHA standards is an important part of
training for electrical apprentices. See the appendix for a list of relevant standards.
Always lock out and tag out
circuits.
• Take steps to control hazards:
• Create a safe workplace.
• Work safely.
Safety Model Stage 1— Recognizing
Hazards
How Do You Recognize Hazards?
The first step toward protecting yourself is
recognizing the many hazards you face on the
job. To do this, you must know which situa-tions
can place you in danger. Knowing where
to look helps you to recognize hazards.
Workers face many hazards on the job:
• Inadequate wiring is dangerous.
• Exposed electrical parts are dangerous.
• Overhead powerlines are dangerous.
• Wires with bad insulation can give you a
shock.
• Electrical systems and tools that are not grounded or double- insulated are dangerous.
• Overloaded circuits are dangerous.
• Damaged power tools and equipment are electrical hazards.
• Using the wrong PPE is dangerous.
• Using the wrong tool is dangerous.
• Some on- site chemicals are harmful.
• Defective ladders and scaffolding are dangerous.
• Ladders that conduct electricity are dangerous.
• Electrical hazards can be made worse if the worker, location or equipment is wet.
Inadequate Wiring Hazards
An electrical hazard exists when the wire is too small a gauge
for the current it will carry. Normally, the circuit breaker in a cir-cuit
is matched to the wire size. However, in older wiring, branch
lines to permanent ceiling light fixtures could be wired with a
smaller gauge than the supply cable. Let’s say a light fixture is
replaced with another device that uses more current. The current
capacity ( ampacity) of the branch wire could be exceeded. When
a wire is too small for the current it is supposed to carry, the wire
will heat up. The heated wire could cause a fire. Again, keep in
mind and consider the following wiring hazards to ensure proper
safety:
29
Use the safety model to recognize, evaluate, and control workplace hazards
like those in this picture.
An electrician was removing a metal fish tape from a hole at the base of a metal light pole. ( A fish tape is used to pull
wire through a conduit run.) The fish tape became energized, electrocuting him. As a result of its inspection, OSHA
issued a citation for three serious violations of the agency’s construction standards. If the following OSHA require-ments
had been followed, this death could have been prevented.
• De- energize all circuits before beginning work.
• Always lock out and tag out de- energized equipment.
• Companies must train workers to recognize and avoid unsafe conditions associated with their work.
Worker was electrocuted while
removing energized fish tape.
Fish tape.
• Wire gauge— wire size or diameter ( technically, the cross- sectional area).
• Ampacity— the maximum amount of current a wire can carry safely without overheating.
• Overloaded wires get hot.
When you use an extension cord, the size of the wire you are placing into the circuit may be too small for the equip-ment.
The circuit breaker could be the right size for the circuit but not right for the smaller- gauge extension cord. A tool
plugged into the extension cord may use more current than the cord can handle without tripping the circuit breaker. The
wire will overheat and could cause a fire. The kind of metal used as a conductor can cause an electrical hazard. Special
care needs to be taken with aluminum wire. Since it is more brittle than copper, aluminum wire can crack and break more
easily. Connections with aluminum wire can become loose and oxidize if not made properly, creating heat or arcing. You
need to recognize that inadequate wiring is a hazard. Incorrect wiring practices can cause fires.
Exposed electrical parts hazards
Electrical hazards exist when wires or other electrical parts are exposed. Wires and
parts can be exposed if a cover is removed from a wiring or breaker box. The overhead
wires coming into a home may be exposed. Electrical terminals in motors, appliances and
electronic equipment may be exposed. Older equipment may have exposed electrical
parts. If you contact exposed live electrical parts, you will be shocked. You need to recog-nize
that an exposed electrical component is a hazard.
• If you touch live electrical parts, you will be shocked.
Overhead Power Line Hazards
Most people do not realize that overhead power lines are usually not insulated. More
than half of all electrocutions are caused by direct worker contact with energized power
lines. Power line workers must be especially aware of the dangers of overhead lines. In
the past, 80 percent of all lineman deaths were caused by contacting a live wire with a
bare hand. Due to such incidents, all linemen now wear special rubber gloves that protect
them up to 34,500 volts. Today, most electrocutions involving overhead power lines are
caused by failure to maintain proper work distances.
• Overhead power lines kill many workers!
Shocks and electrocutions occur where physical barriers are not in place to prevent contact with the wires. When dump
trucks, cranes, work platforms or other conductive materials ( such as pipes and ladders) contact overhead wires, the
equipment operator or other workers can be killed. If you do not maintain required clearance distances from power lines,
you can be shocked and killed. ( The minimum distance for voltages up to 50 kV is 10 feet. For voltages over 5 kV, the
minimum distance is 10 feet plus 4 inches for every 10 kV over 50 kV.) Never store materials and equipment under or
near overhead power lines. You need to recognize that overhead power lines are a hazard.
30
This hand- held sander has
exposed wires and should not
be used.
Watch out for exposed
electrical wires around
electronic equipment.
Electrical line workers
need special training and
equipment to work safely.
Operating a crane near overhead wires is
very hazardous.
Example of overhead power lines hazard
Defective Insulation Hazards
Insulation that is defective or inadequate is an electrical hazard. Usually, a plastic or rubber cover-ing
insulates wires. Insulation prevents conductors from coming in contact with each other. Insulation
also prevents conductors from coming in contact with people.
• Insulation— material that does not conduct electricity easily.
Extension cords may have damaged insulation. Sometimes the insulation inside an electrical tool or
appliance is damaged. When insulation is damaged, exposed metal parts may become energized if a
live wire inside touches them. Electric hand tools that are old, damaged or misused may have damaged
insulation inside. If you touch damaged power tools or other equipment, you will receive a shock. You
are more likely to receive a shock if the tool is not grounded or double- insulated. ( Double- insulated
tools have two insulation barriers and no exposed metal parts.) You need to recognize that defective
insulation is a hazard.
• If you touch a damaged live power tool, you will be shocked.
• A damaged live power tool that is not grounded or double insulated is very dangerous.
Improper Grounding Hazards
When an electrical system is not grounded properly, a hazard exists. The most common OSHA
electrical violation is improper grounding of equipment and circuitry. The metal parts of an electrical
wiring system that we touch ( switch plates, ceiling light fixtures, conduit, etc.) should be grounded
and at 0 volts. If the system is not grounded properly, these parts may become energized. Metal parts
of motors, appliances or electronics that are plugged into improperly grounded circuits may be energized. When a circuit
is not grounded properly, a hazard exists because unwanted voltage cannot be safely eliminated. If there is no safe path to
ground for fault currents, exposed metal parts in damaged appliances can become energized. Extension cords may not
provide a continuous path to ground because of a broken ground wire or plug. If you contact a defective electrical device
that is not grounded ( or grounded improperly), you will be shocked. You need to recognize that an improperly grounded
electrical system is a hazard.
• Fault current— any current that is not in its intended path.
• Ground potential— the voltage a grounded part should have; 0 volts relative to ground.
• If you touch a defective live component that is not grounded, you will be shocked.
Electrical systems are often grounded to metal water pipes that serve as a continuous path to ground. If plumbing is
used as a path to ground for fault current, all pipes must be made of conductive material ( a type of metal). Many electro-cutions
and fires occur because ( during renovation or repair) parts of metal plumbing are replaced with plastic pipe,
which does not conduct electricity. In these cases, the path to ground is interrupted by nonconductive material. A ground
31
Five workers were constructing a chain- link fence in front of a house, directly below a 7,200- volt energized power line.
As they prepared to install 21- foot sections of metal top rail on the fence, one of the workers picked up a section of rail
and held it up vertically. The rail contacted the 7,200- volt line, and the worker was electrocuted. Following inspection,
OSHA determined that the employee who was killed had never received any safety
training from his employer and no specific instruction on how to avoid the hazards
associated with overhead power lines.
In this case, the company failed to obey these regulations:
• Employers must train their workers to recognize and avoid unsafe conditions on the
job.
• Employers must not allow their workers to work near any part of an electrical circuit
UNLESS the circuit is de- energized ( shut off) and grounded, or guarded in such a
way that it cannot be contacted.
• Ground- fault protection must be provided at construction sites to guard against elec-trical
shock.
This extension
cord is damaged
and should not
be used.
fault circuit interrupter, or GFCI, is an inexpensive life- saver. GFCI’s detect any difference in current between the two cir-cuit
wires ( the black wires and white wires). This difference in current could happen when electrical equipment is not
working correctly, causing leakage current. If leakage current ( a ground fault) is detected in a GFCI- protected circuit, the
GFCI switches off the current in the circuit, protecting you from a dangerous shock. GFCI’s are set at about 5 mA and are
designed to protect workers from electrocution. GFCI’s are able to detect the loss of current resulting from leakage
through a person who is beginning to be shocked. If this situation occurs, the GFCI switches off the current in the circuit.
GFCI’s are different from circuit breakers because they detect leakage currents rather than overloads. Circuits with miss-ing,
damaged, or improperly wired GFCI’s may allow you to be shocked. You need to recognize that a circuit improperly
protected by a GFCI is a hazard.
• GFCI— ground fault circuit interrupter- a device that detects current leakage from a cir-cuit
to ground and shuts the current off.
• Leakage current— current that does not return through the intended path but instead
“ leaks” to ground.
• Ground fault��� a loss of current from a circuit to a ground connection.
Overload Hazards
Overloads in an electrical system are hazardous because they can produce heat or arcing. Wires and other components
in an electrical system or circuit have a maximum amount of current they can carry safely. If too many devices are
plugged into a circuit, the electrical current will heat the wires to a very high temperature. If any one tool uses too much
current, the wires will heat up.
• Overload— too much current in a circuit.
• An overload can lead to a fire or electrical shock.
The temperature of the wires can be high enough to cause a fire. If their insulation melts,
arcing may occur. Arcing can cause a fire in the area where the overload exists, even inside a
wall. In order to prevent too much current in a circuit, a circuit breaker or fuse is placed in the
circuit. If there is too much current in the circuit, the breaker “ trips” and opens like a switch. If
an overloaded circuit is equipped with a fuse, an internal part of the fuse melts, opening the
circuit. Both breakers and fuses do the same thing: open the circuit to shut off the electrical
current. If the breakers or fuses are too big for the wires they are supposed to protect, an over-load
in the circuit will not be detected and the current will not be shut off. Overloading leads to
overheating of circuit components ( including wires) and may cause a fire. You need to recognize that a circuit with
improper overcurrent protection devices— or one with no overcurrent protection devices at all— is a hazard.
• Circuit breaker— an overcurrent protection device that automatically shuts off the current in a circuit if an overload
occurs.
• Trip— the automatic opening ( turning off) of a circuit by a GFCI or circuit breaker.
• Fuse— an overcurrent protection device that has an internal part that melts and shuts off the current in a circuit if
there is an overload.
• Circuit breakers and fuses that are too big for the circuit are dangerous.
• Circuits without circuit breakers or fuses are dangerous.
Overcurrent protection devices are built into the wiring of some electric motors, tools and electronic devices. For
example, if a tool draws too much current or if it overheats, the current will be shut off from within the device itself.
Damaged tools can overheat and cause a fire. You need to recognize that a damaged tool is a hazard.
• Damaged power tools can cause overloads.
Wet Conditions Hazards
Working in wet conditions is hazardous because you may become an easy path for electrical current. If you touch a live
wire or other electrical component— and you are well- grounded because you are standing in even a small puddle of
water— you will receive a shock.
• Wet conditions are dangerous.
32
GFCI receptacle
Overloads are a major
cause of fires.
Damaged insulation, equipment or tools can expose you to live electrical parts. A damaged tool may not be grounded
properly, so the housing of the tool may be energized, causing you to receive a shock. Improperly grounded metal switch
plates and ceiling lights are especially hazardous in wet conditions. If you touch a live electrical component with an unin-sulated
hand tool, you are more likely to receive a shock when standing in water. But remember: you don’t have to be
standing in water to be electrocuted. Wet clothing, high humidity and perspiration also increase your chances of being
electrocuted. You need to recognize that all wet conditions are hazards.
• An electrical circuit in a damp place without a GFCI is dangerous. A GFCI reduces the danger.
Additional Hazards
In addition to electrical hazards, other types of hazards are present at job sites. Remember that
all of these hazards can be controlled.
• There may be chemical hazards. Solvents and
other substances may be poisonous or cause dis-ease.
• Frequent overhead work can cause tendinitis
( inflammation) in your shoulders.
• Intensive use of hand tools that involve force or
twisting can cause tendinitis of the hands, wrists
or elbows. Use of hand tools can also cause
carpal tunnel syndrome, which results when
nerves in the wrist are damaged by swelling ten-dons
or contracting muscles.
Examples of Additional Hazards ( Non- electrical)
• PPE— personal protective equipment ( eye protection, hard hat, special clothing, etc.)
• Low back pain can result from lifting objects the wrong way or carrying heavy loads of wire
or other material. Back pain can also occur as a result of injury from poor working surfaces
such as wet or slippery floors. Back pain is common, but it can be disabling and can affect
young individuals.
• Chips and particles flying from tools can injure your eyes. Wear eye protection.
• Falling objects can hit you. Wear a hard hat.
• Sharp tools and power equipment can cause cuts and other injuries. If you receive a shock,
you may react and be hurt by a tool.
33
Overhead work can
cause long- term
shoulder pain.
Frequent use of some hand tools can
cause wrist problems such as carpal
tunnel syndrome.
A 22- year- old carpenter’s apprentice was killed when he was struck in the head by a nail fired from a powder- actuated
nail gun ( a device that uses a gun powder cartridge to drive nails into concrete or steel). The nail gun operator fired
the gun while attempting to anchor a plywood concrete form, causing the nail to pass through the hollow form. The nail
traveled 27 feet before striking the victim. The nail gun operator had never received training on how to use the tool,
and none of the employees in the area was wearing PPE.
In another situation, two workers were building a wall while remodeling a house. One of the workers was killed when
he was struck by a nail fired from a powder- actuated nail gun. The tool operator who fired the nail was trying to attach
a piece of plywood to a wooden stud. But the nail shot though the plywood and stud, striking the victim. Below are
some OSHA regulations that should have been followed.
• Employees using powder- or pressure- actuated tools must be trained to use them safely.
• Employees who operate powder- or pressure- actuated tools must be trained to avoid firing into easily penetrated
materials ( like plywood).
• In areas where workers could be exposed to flying nails, appropriate PPE must be used.
Lift with your legs,
not your back!
• You can be injured or killed by falling from a ladder or scaffolding. If you receive a shock—
even a mild one— you may lose your balance and fall. Even without being shocked, you
could fall from a ladder or scaffolding.
• You expose yourself to hazards when you do not wear PPE.
All of these situations need to be recognized as hazards.
Summary
You need to be able to recognize that electrical shocks, fires, or falls result from these hazards:
• Inadequate wiring
• Exposed electrical parts
• Overhead powerlines
• Defective insulation
• Improper grounding
• Overloaded circuits
• Wet conditions
• Damaged tools and equipment
• Improper PPE
Safety Model Stage 2— Evaluating Hazards
How Do You Evaluate Your Risk?
After you recognize a hazard, your next step is to evaluate your risk from the hazard. Obviously, exposed wires should
be recognized as a hazard. If the exposed wires are 15 feet off the ground, your risk is low. However, if you are going to
be working on a roof near those same wires, your risk is high. The risk of shock is greater if you will be carrying metal
conduit that could touch the exposed wires. You must constantly evaluate your risk.
• Risk— the chance that injury or death will occur.
• Make the right decisions.
Combinations of hazards increase your risk. Improper grounding and a dam-aged
tool greatly increase your risk. Wet conditions combined with other hazards
also increase your risk. You will need to make decisions about the nature of haz-ards
in order to evaluate your risk and do the right thing to remain safe. There are
clues that electrical hazards exist. For example, if a GFCI keeps tripping while
you are using a power tool, there is a problem. Don’t keep resetting the GFCI and
continuing to work. You must evaluate the clue and decide what action should be
taken to control the hazard. There are a number of other conditions that indicate a
hazard.
• Short— a low- resistance path between a live wire and the ground, or
between wires at different voltages ( called a fault if the current is unintended).
• Tripped circuit breakers and blown fuses show that too much current is flowing in a circuit. This condition could be
due to several factors, such as malfunctioning equipment or a short between conductors. You need to determine the
cause in order to control the hazard.
• An electrical tool, appliance, wire or connection that feels warm may indicate too much current in the circuit or
equipment. You need to evaluate the situation and determine your risk.
• An extension cord that feels warm may indicate too much current for the wire size of the cord. You must decide
when action needs to be taken.
• A cable, fuse box or junction box that feels warm may indicate too much current in the circuits.
• A burning odor may indicate overheated insulation.
34
You need to be espe-cially
careful when
working on scaffold-ing
or ladders.
Combinations of hazards increase risk.
• Worn, frayed or damaged insulation around any wire or other conductor is an electrical hazard because the conduc-tors
could be exposed. Contact with an exposed wire could cause a shock. Damaged insulation could cause a short,
leading to arcing or a fire. Inspect all insulation for scrapes and breaks. You need to evaluate the seriousness of any
damage you find and decide how to deal with the hazard.
• A GFCI that trips indicates there is current leakage from the circuit. First you must decide the probable cause of the
leakage by recognizing any contributing hazards. Then you must decide what action needs to be taken.
Summary
• Look for “ clues” that hazards are present.
• Evaluate the seriousness of hazards.
• Decide if you need to take action.
• Don’t ignore signs of trouble.
Safety Model Stage 3— Controlling Hazards: Safe Work Environment
How Do You Control Hazards?
In order to control hazards, you must first create a safe work environment, then work in a safe manner. Generally, it is
best to remove the hazards altogether and create an environment that is truly safe. When OSHA regulations and the NEC
are followed, safe work environments are created. But you never know when materials or equipment might fail. Prepare
yourself for the unexpected by using safe work practices. Use as many safeguards as possible. If one fails, another may
protect you from injury or death.
How Do You Create a Safe Work Environment?
A safe work environment is created by controlling contact with electrical voltages and the currents they can cause.
Electrical currents need to be controlled so they do not pass through the body. In addition to preventing shocks, a safe
work environment reduces the chance of fires, burns and falls. You need to guard against contact with electrical voltages
and control electrical currents in order to create a safe work environment. Make your environment safer by doing the fol-lowing:
• Treat all conductors— even “ de- energized” ones— as if they are energized until they are locked out and tagged.
• Lock out and tag out circuits and machines.
• Prevent overloaded wiring by using the right size and type of wire.
• Prevent exposure to live electrical parts by isolating them.
• Prevent exposure to live wires and parts by using insulation.
• Prevent shocking currents from electrical systems and tools by grounding them.
• Prevent shocking currents by using GFCIs.
• Prevent too much current in circuits by using overcurrent protection devices.
Lock Out and Tag Out Circuits and Equipment
Create a safe work environment by locking out and tagging out circuits and machines. Before working on a circuit, you
must turn off the power supply. Once the circuit has been shut off and de- energized, lock out the switchgear to the circuit
so the power cannot be turned back on inadvertently. Then tag out the circuit with an easy- to- see sign or label that lets
everyone know that you are working on the circuit. If you are working on or near machinery, you must lock out and tag
out the machinery to prevent startup. Before you begin work, you must test the circuit to make sure it is de- energized.
35
Lockout/ Tagout Checklist
Lockout/ tagout is an essential safety procedure that protects work-ers
from injury while working on or near electrical circuits and equip-ment.
Lockout involves applying a physical lock to the power
source( s) of circuits and equipment after they have been shut off and
de- energized. The source is then tagged out with an easy- to- read tag
that alerts other workers in the area that a lock has been applied.
Disconnecting means required by Subjpart S must be capable of
accepting a lock ( consistent with 1910.147( c)( 2)( iii)).
In addition to protecting workers from electrical hazards, lockout/ tagout prevents contact with
operating equipment parts: blades, gears, shafts, presses, etc. When performing lockout/ tagout on
circuits and equipment, you can use the checklist items below as a guide.
• Identify all sources of electrical energy for the equipment or circuits in question.
• Disable backup energy sources such as generators and batteries.
• Identify all shutoffs for each energy source.
• Notify all personnel that equipment and circuitry must be shut off, locked out and tagged out. ( Simply turning a
switch off is NOT enough.)
• Shut off energy sources and lock switchgear in the OFF position. Each worker should apply his or her individual
lock. Do not give your key to anyone.
• Test equipment and circuitry to make sure they are de- energized. This must be done by an authorized person. ( An
“ authorized” person is defined as someone who has received required training on the hazards and on the construc-tion
and operation of equipment involved in a task.) ( See 1910.147( b) as applicable.)
• Deplete stored energy by bleeding, blocking, grounding, etc.
�� Apply a tag to alert other workers that an energy source or piece of equipment has been locked out.
• Make sure everyone is safe and accounted for before equipment and circuits are unlocked and turned back on. Note
that only an authorized person may determine when it is safe to re- energize circuits.
Control Inadequate Wiring Hazards
Electrical hazards result from using the wrong size or type of wire. You must control such hazards to create a safe work
environment. You must choose the right size wire for the amount of current expected in a circuit. The wire must be able to
handle the current safely. The wire’s insulation must be appropriate for the voltage and tough enough for the environment.
Connections need to be reliable and protected.
• Use the right size and type of wire.
• AWG— American Wire Gauge— a measure of wire size.
36
Always test a circuit to make sure it is
de- energized before working on it. Lockout/ tagout saves lives.
Control Hazards of Fixed Wiring
The wiring methods and size of conductors used in a system depend on several factors:
• Intended use of the circuit system
• Building materials
• Size and distribution of electrical load
• Location of equipment ( such as underground burial)
• Environmental conditions ( such as dampness)
• Presence of corrosives
• Temperature extreme
Fixed, permanent wiring is better than extension cords, which can be misused and
damaged more easily. NEC requirements for fixed wiring should always be followed. A
variety of materials can be used in wiring applications, including nonmetallic sheathed
cable ( Romex ® ), armored cable, and metal and plastic conduit. The choice of wiring
material depends on the wiring environment and the need to support and protect wires.
• Fixed wiring— the permanent wiring installed in homes and other buildings.
Aluminum wire and connections should be handled with special care. Connections
made with aluminum wire can loosen due to heat expansion and oxidize if they are not
made properly. Loose or oxidized connections can create heat or arcing. Special clamps
and terminals are necessary to make proper connections using aluminum wire.
Antioxidant paste can be applied to connections to prevent oxidation.
37
Wires come in different sizes. The maximum current each size can conduct safely is shown.
Nonmetallic sheathing helps
protect wires from damage.
Control Hazards of Flexible Wiring
Use Flexible Wiring Properly
Electrical cords supplement fixed wiring by providing the flexibility required for maintenance, portability, isolation
from vibration, and emergency and temporary power needs. Flexible wiring can be used for extension cords or power
supply cords. Power supply cords can be removable or permanently attached to the appliance.
• Flexible wiring— cables with insulated and stranded wire that bends easily.
DO NOT use flexible wiring in situations where frequent inspection would be difficult, where damage would be likely,
or where long- term electrical supply is needed. Flexible cords cannot be used as a substitute for the fixed wiring of a
structure. Flexible cords must not be:
• Run through holes in walls, ceilings or floors.
• Run through doorways, windows or similar openings ( unless physically protected).
• Attached to building surfaces ( except with a tension take- up device within 6 feet of the supply end).
• Hidden in walls, ceilings or floors.
• Hidden in conduit or other raceways.
Use the Right Extension Cord
The size of wire in an extension cord must be compatible with the amount of current the cord will be expected to carry.
The amount of current depends on the equipment plugged into the extension cord. Current ratings ( how much current a
device needs to operate) are often printed on the nameplate. If a power rating is given, it is necessary to divide the power
rating in watts by the voltage to find the current rating. For example, a 1,000- watt heater plugged into a 120- volt circuit
will need almost 10 amps of current. Let’s look at another example: A 1- horsepower electric motor uses electrical energy
at the rate of almost 750 watts, so it will need a minimum of about 7 amps of current on a 120- volt circuit. But electric
motors need additional current as they startup or if they stall, requiring up to 200 percent of the

Click tabs to swap between content that is broken into logical sections.

A Guide to
Electrical Safety
N. C. Department of Labor
Occupational Safety and Health Division
1101 Mail Service Center
Raleigh, NC 27699- 1101
Cherie Berry
Commissioner of Labor
18
N. C. Department of Labor
Occupational Safety and Health Program
Cherie Berry
Commissioner of Labor
OSHA State Plan Designee
Allen McNeely
Deputy Commissioner for Safety and Health
Kevin Beauregard
Assistant Deputy Commissioner for Safety and Health
Edward E. Lewis
Reviewer
Acknowledgments
A Guide to Electrical Safety was prepared by Ed Mendenhall of Mendenhall Technical Services with additional materi-als
provided by N. C. Department of Labor employee Dwight Grimes. The information in this guide was updated in
2008.
This guide is intended to be consistent with all existing OSHA standards; therefore, if an area is considered by the
reader to be inconsistent with a standard, then the OSHA standard should be followed.
To obtain additional copies of this guide, or if you have questions about North Carolina occupational safety and health stan-dards
or rules, please contact:
N. C. Department of Labor
Education, Training and Technical Assistance Bureau
1101 Mail Service Center
Raleigh, NC 27699- 1101
Phone: ( 919) 807- 2875 or 1- 800- NC- LABOR
____________________
Additional sources of information are listed on the inside back cover of this guide.
____________________
The projected cost of the NCDOL OSH program for federal fiscal year 2009– 2010 is $ 17,534,771. Federal funding provides approximately 30 percent ($ 5,180,700) of
this fund.
Revised 2/ 08
Contents
Part Page
Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1iiv
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ivi1
2 Fundamentals of Electricity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii14
3 Arc Flash/ NFPA 70E . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii11
4 Branch Circuit Wiring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii13
5 Branch Circuit and Equipment Testing . . . . . . . . . . . . . . . . . . . . . . . . . ii21
6 Voltage Detector Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii24
7 Ground Fault Circuit Interrupters . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii26
8 Common Electrical Deficiencies . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii43
9 Inspection Guidelines/ Checklist . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii57
10 Safety Program Policy and Procedures . . . . . . . . . . . . . . . . . . . . . . . . . ii59
iii
Foreword
Everyone from office clerks to farmers work around electricity on a daily basis. Our world is filled with overhead
power lines, extension cords, electronic equipment, outlets and switches. Our access to electricity has become so common
that we tend to take our safety for granted. We forget that one frayed power cord or a puddle of water on the floor can
take us right into the electrical danger zone.
A Guide to Electrical Safety can help electricians, plant maintenance personnel and many others review safe proce-dures
for electrical work. It also covers the main U. S. Occupational Safety and Health Administration standards concern-ing
electrical safety on the job.
In North Carolina, state inspectors enforce the federal laws through a state plan approved by the U. S. Department of
Labor. The N. C. Department of Labor is charged with this mission. NCDOL enforces all current OSHA standards. It
offers many educational programs to the public and produces publications, including this guide, to help inform people
about their rights and responsibilities.
When reading this guide, please remember the NCDOL mission is greater than enforcement of regulations. An equally
important goal is to help citizens find ways to create safer workplaces. A Guide to Electrical Safety can help you make
and keep your workplace free of dangerous electrical hazards.
Cherie Berry
Commissioner of Labor
v
1
1
Introduction
Electricity is the modern version of the genie in Aladdin’s lamp. When electricity is safely contained in an insulated
conductor, we normally cannot see, smell, taste, feel or hear it. It powers an endless list of laborsaving appliances and
life- enhancing and support systems that have become such an assumed part of our lives that we give little thought to its
potential for causing harm. Many myths and misstatements about electrical action are accepted as fact by many people.
The National Institute for Occupational Safety and Health ( NIOSH) conducted a study of workplace electrocutions that
revealed the following information about workers who were electrocuted:
• The average age was 32.
• 81 percent had a high school education.
• 56 percent were married.
• 40 percent had less than one year of experience on the job to which they were assigned at the time of the fatal acci-dent.
• 96 percent of the victims had some type of safety training, according to their employers.
This information reminds us that more effective training and education must be provided to employees if we are to
reduce workplace electrocution hazards. Employees should receive initial training then refresher electrical hazard recog-nition
training on an annual basis.
In addition to the shock and electrocution hazards, electricity can also cause fires and explosions. According to the U. S.
Consumer Product Safety Commission, an estimated 169,000 house fires of electrical origin occur each year, claiming
1,100 lives and injuring 5,600 people. Property losses from fires begun by electricity are estimated at $ 1.1 billion each
year. The safe use and maintenance of electrical equipment at work ( and at home) will help prevent fire and physical injury.
This guide provides a clear understanding of electrical action and its control in the workplace environment. This infor-mation
will enable you to recognize electrical hazards in the workplace as well as provide information on their control
and/ or elimination. The guide does not qualify a person to work on or near exposed energized parts. Training
requirements for “ qualified” persons ( those permitted to work on or near exposed energized parts) are detailed in
29 CFR 1910.332( b)( 3). Also, 29 CFR 1910.399, Definitions Applicable to Subpart S gives a definition of “ qualified
person.” The guide will, however, enhance your ability to find and report electrical deficiencies in need of a quali-fied
person’s attention.
Dangers of Electricity
Whenever you work with power tools or on electrical circuits, there is a risk of electrical hazards, especially electrical
shock. Anyone can be exposed to these hazards at home or at work. Workers are exposed to more hazards because job-sites
can be cluttered with tools and materials, fast- paced, and open to the weather. Risk is also higher at work because
many jobs involve electric power tools.
Electrical trades workers must pay special attention to electrical hazards because they work on electrical circuits.
Coming in contact with an electrical voltage can cause current to flow through the body, resulting in electrical shock and
burns. Serious injury or even death may occur. As a source of energy, electricity is used without much thought about the
hazards it can cause. Because electricity is a familiar part of our lives, it often is not treated with enough caution. As a
result, an average of one worker is electrocuted on the job every day of every year. Electrocution is the third leading cause
of work- related deaths among 16- and 17- year- olds, after motor vehicle deaths and workplace homicide. Electrocution is
the cause of 12 percent of all workplace deaths among young workers. 1
____________
1 Castillo D. N. [ 1995]. NIOSH Alert: Preventing Death and Injuries of Adolescent Workers. Cincinnati, Ohio: U. S. Department of Health and Human Services, Public
Health Service, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, DHHS ( NIOSH) Publication No. 95- 125.
• Electrical shock causes injury or death!
• current— the movement of electrical charge
• voltage— a measure of electrical force
• circuit— a complete path for the flow of current
• You will receive a shock if you touch two wires at different volt-ages
at the same time.
This industry guide offers discussion on a variety of topics as pertained
to electrical hazards. There are four main types of electrical injuries: elec-trocution
( death due to electrical shock), electrical shock, burns and
falls. The guide discusses the dangers of electricity, electrical shock and
the resulting injuries. It describes the various electrical hazards. The guide includes a sample plan ( Safety Model) or
approach to address these hazards in a later section. ( This sample model/ approach is also useful with other hazards.) You
will learn about the Safety Model, as an important tool for recognizing, evaluating and controlling hazards. The guide
includes important definitions and notes are shown throughout. It emphasizes practices that will help keep you safe and
free of injury. It also includes case studies about real- life deaths to give you an idea of the hazards caused by electricity.
How Is an Electrical Shock Received?
An electrical shock is received when electrical current passes through the body. Current will pass through the body in a
variety of situations. Whenever two wires are at different voltages, current will pass between them if they are connected.
Your body can connect the wires if you touch both of them at the same time. Current will pass through your body.
• ground— a physical electrical connection to the earth
• energized ( live, “ hot”)— similar terms meaning that a voltage is present that can cause a
current, so there is a possibility of getting shocked
In most household wiring, the black wires and the red wires are at 120 volts. The white wires are
at 0 volts because they are connected to ground. The connection to ground is often through a con-ducting
ground rod driven into the earth. The connection can also be made through a buried metal
water pipe. If you come in contact with an energized black wire— and you are also in contact with
the neutral white wire— current will pass through your body. You will receive an electrical shock.
• conductor— material in which an electrical current moves easily
• neutral— at ground potential ( 0 volts) because of a connection to ground
If you are in contact with a live wire or any live component
of an energized electrical device— and also in contact with any
grounded object— you will receive a shock. Plumbing is often
grounded. Metal electrical boxes and conduit are grounded.
Your risk of receiving a shock is greater if you stand in a
puddle of water. But you don’t even have to be standing in
water to be at risk. Wet clothing, high humidity and perspiration
also increase your chances of being shocked. Of course, there is
always a chance of shock, even in dry conditions. You can even
receive a shock when you are not in contact with an electrical
ground. Contact with both live wires of a 240- volt cable will
deliver a shock. ( This type of shock can occur because one live
wire may be at + 120 volts while the other is at – 120 volts dur-ing
an alternating current cycle— a difference of 240 volts.) You
can also receive a shock from electrical components that are not
grounded properly. Even contact with another person who is
receiving an electrical shock may cause you to be shocked.
2
Electrical work can be deadly if not done safely.
Wires carry current.
Metal electrical boxes should
be grounded to prevent shocks.
Black and red wires are
usually energized, and
white wires are usually
neutral.
• You will receive a shock if you touch a live wire and are grounded at the same time.
• When a circuit, electrical component or equipment is energized, a potential shock haz-ard
is present.
Summary
You will receive an electrical shock if a part of your body completes an electrical circuit by
touching a live wire and an electrical ground, or touching a live wire and another wire at a differ-ent
voltage.
3
Always test a circuit
to make sure it is
de- energized before
working on it.
2
Fundamentals of Electricity
A review of the fundamentals of electricity is necessary to an understanding of some common myths and misstate-ments
about electricity. First we must review Ohm’s Law and understand the effects of current on the human body. Basic
rules of electrical action will enhance your ability to analyze actual or potential electrical hazards quickly. This informa-tion
will also enable you to understand other important safety concepts such as reverse polarity, equipment grounding,
ground fault circuit interrupters, double insulated power tools, and testing of circuits and equipment.
Ohm’s Law
There are three factors involved in electrical action. For electrons to be activated or caused to flow, those three factors
must be present. A voltage ( potential difference) must be applied to a resistance ( load) to cause current to flow when there
is a complete loop or circuit to and from the voltage source. Ohm’s Law simply states that 1 volt will cause a current of 1
ampere to flow through a resistance of 1 ohm. As a formula this is stated as follows:
Voltage ( E) = Current ( I) X Resistance ( R).
We will be concerned about the effects of current on the human body, so the formula relationship we will use most will
be I = E/ R. When you analyze reported shock hazards or electrical injuries, you should look for a voltage source and a
resistance ( high or low) ground loop. The human body is basically a resistor and its resistance can be measured in ohms.
Figure 1 depicts a body resistance model. The resistive values are for a person doing moderate work. An increase in per-spiration
caused from working at a faster work pace would decrease the resistance and allow more current to flow.
As an example, let’s use the hand to hand resistance of the body model, 500 + 500
= 1,000 ohms. Using I = E/ R, I = 120/ 1,000 ( assuming a 120 volt AC ( alternating
current) power source) or 0.120 amps. If we multiply 0.120 amps by 1,000 ( this con-verts
amps to milliamps), we get 120 milliamps ( mA) which we will refer to in
Figure 2. If a person were working in a hot environment, and sweating, the body
resistance could be lowered to a value of 500 ohms. Then the current that could flow
through the body would equal I = 120/ 500 or 0.240 amps. Changing this to mil-liamps,
1,000 X 0.240 = 240 mA. This means that we have doubled the hazard to the
body by just doing our job.
This can be explained by looking at Figure 2. Figure 2 plots the current flowing
through the chest area and the time it takes to cause the heart to go into ventricular
fibrillation
( arrhythmic
heartbeat).
Using the
example of
the body
resistance at
1,000 ohms allowing 120 mA to flow ( follow
the dark line vertically from 120 mA to the
shaded area, then left to the time of 0.8 seconds),
you can see that it would only take 0.8 seconds
to cause electrocution. When the body resistance
is 500 ohms, at 240 mA it would only take 0.2
seconds to cause electrocution. Variable condi-tions
can make common- use electricity ( 110
volts, 15 amps) fatal.
4
Figure 1
Human Body Resistance Model
10.0
6.0
2.0
1.0
0.6
.2
.1
.06
.02
.01
Time In Seconds
‘ Let Go’
Range
Maximum Permitted
By UL For Class A GFCI
Electrocution Threshold
For Typical Adult
0 20 40 60 80 100 120 140 160 180 200 220 240 260
Current In Milliamperes
Figure 2
Electrical Current ( AC) Versus the Time It Flows Through the Body
Current and Its Effect on the Human Body
Based on the research of Professor Dalziel of the University of California, Berkeley, the effect of 60 Hz ( cycles per
second) of alternating current on the human body is generally accepted to be as follows:
• 1 milliamp ( mA) or less— no sensation— not felt ( 1,000 milliamps equal 1 amp)
• 3 mA or more painful shock
• 5 mA or more— local muscle contractions— 50 percent cannot let go
• 30 mA or more— breathing difficult— can cause unconsciousness
• 50– 100 mA— possible heart ventricular fibrillation
• 100– 200 mA— certain heart ventricular fibrillation
• 200 mA or more— severe burns and muscular contractions— heart more apt to stop than fibrillate
• Over a few amps— irreversible body damage
Thus, we can see that there are different types of injuries that electricity can cause. At the 20 to 30 mA range a form of
anoxia ( suffocation) can result. This could happen in a swimming pool where there is a ground loop present ( the drain at the
bottom of the pool) if a faulty light fixture or appliance is dropped into the water. Current would flow from the light fixture
to the drain, using the water as the conducting medium. Any person swimming through the electrical field created by the
fault current would be bathed in potential difference, and the internal current flow in the body could paralyze the breathing
mechanism. This is why it is very important to keep all portable electrical appliances away from sinks, tubs and pools.
Ventricular fibrillation generally can occur in the range of 50 to 200 mA. Ventricular fibrillation is the repeated, rapid,
uncoordinated contractions of the ventricles of the heart resulting in the loss of synchronization between the heartbeat and
the pulse beat. Once ventricular fibrillation occurs, death can ensue in a few minutes. Properly applied CPR ( cardiopul-monary
resuscitation) techniques can save the victim until emergency rescue personnel with a defibrillator arrive at the
scene. Workers in the construction trades and others working with electrical power tools should receive CPR training.
Above a few amperes, irreversible body damage can occur. This condition is more likely to occur at voltages above
600 volts AC. For example, if a person contacted 10,000 volts, I = 10,000/ 1,000 = 10 amps. This amount of current would
create a great amount of body heat. Since the body consists of over 60 percent water, the water would turn to steam at a
ratio of approximately 1 to 1,500. This would cause severe burns or exploding of body parts. These are the types of
injuries that you would normally associate with electric power company workers. They can also occur, however, when
people accidentally let a television or radio antenna contact an uninsulated power line. Accidents involving mobile verti-cal
scaffolding or cranes booming up into power lines can cause these types of injuries or fatalities.
The route that the current takes through the body affects the degree of injury. If the current passes through the chest
cavity ( e. g., left hand to right hand), the person is more likely to receive severe injury or electrocution; however, there
have been cases where an arm or leg was burned severely when the extremity came in contact with the voltage and the
current flowed through a portion of the body without going through the chest area of the body. In these cases the person
received a severe injury but was not electrocuted.
Typical 120 Volt AC System
At some time in your life, you
may have received an electrical
shock. Figure 3 illustrates a typical
120 volt AC system. Somewhere
near your home or workplace there
is a transformer with wires going
between the transformer and the ser-vice
entrance panel ( SEP). In small
establishments and homes, the SEP
may also contain circuit breakers or
fuses to protect the circuits leaving
the SEP. Typical overcurrent protec-tion
for these circuits would be 15 or
20 amps. This protection is designed
5
120 Volts
120 Volts
Neutral
Grounding Electrode
Equipment- grounding
Conductor ( green or bare)
Utility
Supply
Service
Ground
Circuit Breaker
Grounded
Conductor
( white or gray)
“ Hot” Conductor
( black or red)
Primary Lines
2.4– 13 kV
Figure 3
AC Systems— Contact With “ Hot” Conductor
for line ( hot) to line ( grounded conductor) faults that would cause current greater than 15 or 20 amps to flow. If a person
accidentally contacted the “ hot��� conductor while standing on the ground with wet feet ( see Figure 3), a severe shock could
result. Current could flow through the body and return to the transformer byway of the “ ground loop” path. Most electrical
shocks result when the body gets into a ground loop and then contacts the “ hot” or ungrounded conductor. If you analyze
electrical shock incidents, look for these two factors: a ground loop and a voltage source.
We normally think of ground as the earth beneath our feet. From an electrical hazard standpoint, ground loops are all
around us. A few ground loops that may not be under our feet include metal water piping, metal door frames in newer
building construction, ventilation ducts, metal sinks, metal T- bars holding ceiling acoustical panels, wet or damp concrete
floors and walls, grounded light fixtures, and grounded power tools/ appliances. When you are using or working around
electrical equipment, be alert to these and other ground loops. The person shown in Figure 3 could have isolated the
ground loop by standing on insulated mats or dry plywood sheets. Wearing dry synthetic soled shoes would also have iso-lated
the ground loop.
The utility supply ground and the grounding electrode conductor are system safety grounds. These grounds protect the
users of electrical equipment in case of lightning storms and in instances where high voltage lines accidentally fall on
lower voltage lines. These system safety grounds are not designed for individual safety. Actually, they are a hazard to the
individual in that it is very easy to get into a ground loop, and once into a ground loop, you only need one fault path to the
hot conductor before shock or injury can result.
Four Principles of Electrical Action
Knowing the basic principles of electrical action will help you understand and evaluate electrical shock hazards. These
principles and an explanation for each are as follows:
1. Electricity does not “ spring” into action until current flows.
2. Current will not flow until there is a loop ( intentionally or accidentally) from the voltage source to a load and back
to the source.
3. Electrical current always returns to the voltage source ( transformer) that created it.
4. When current flows, energy ( measured in watts) results.
Explanation for Principle 1
A person can contact voltage and not be shocked if there is high resistance in the loop. In Figure 3, the person is stand-ing
on the ground and touching the 120 volt conductor. That would cause a shock and make your hair stand on end. If that
same person were standing on insulated mats or wore shoes with insulated soles, the person would not be shocked even
though there was 120 volts in his or her body. This explains why a person can be working outside with a defective power
tool and not receive a shock when the ground is very dry or the person is isolated from a ground loop by plywood. That
same person with the same power tool could change work locations to a wet area, then receive a shock when contacting a
ground loop of low resistance. As previously stated, 3 mA or more can cause painful shock. Using Ohm’s Law I = E/ R,
120 volts and 3 mA, we can calculate how much resistance would allow 3 mA of current to flow. R = E/ I or 120/ 0.003 or
R = 40,000 ohms. Any ground loop resistance of less than 40,000 ohms would allow a shock that could be felt. This prin-ciple
can also explain why birds sitting on a power line are not electrocuted. Their bodies would receive voltage, but cur-rent
would not flow since another part of their body is not in contact with a ground loop.
Explanation for Principle 2
For current to flow, a complete loop must be established from the voltage source to the person and back to the voltage
source. In Figure 3, the loop is through the person’s hand touching a 120 volt conductor, through the body to ground and
then through the grounding electrode and back to the transformer secondary through the neutral conductor. Once that loop
is established and becomes less than 40,000 ohms, a shock or serious injury can result. If the loop can be interrupted, as
noted in Principle 1, then current will not flow. These two principles give you a common sense way to figure out how and
why someone received a shock and the action that should be taken to prevent future shocks of the same type.
Explanation for Principle 3
Electric current always seeks to return to the transformer that created it. Current will also take all resistive paths to
return to the transformer that created it. Since the voltage source has one wire already connected to ground ( Figure 3),
6
contact with the “ hot” wire provides a return path for current to use. Other ground loop paths in the workplace could
include metal ducts, suspended ceiling T- bars, water pipes and other similar ground loops.
Explanation for Principle 4
This principle explains the shock and injury to the human body that current can do. The higher the voltage involved,
the greater the potential heat damage to the body. As previously mentioned, high voltage can cause high current flow
resulting in severe external and internal body damage. Remember that the flow of current causes death or injury; voltage
determines how the injury or death is effected.
Some Misconceptions About Electrical Action
Americans use more electrical power per person than do individuals of any other country in the world, but that does
not mean that we have a better understanding of electricity. Some common misconceptions about electrical actions are
addressed and corrected in the following discussion.
“ If an Appliance or Power Tool Falls Into Water, It Will Short Out���
When an appliance falls into a tub or container of water, it will not short out. In fact, if the appliance switch is “ on,” the
appliance will continue to operate. If the appliance has a motor in it, the air passage to keep the motor cool will be water
cooled. Unfortunately, that same air passage, when wet, will allow electricity to flow outside the appliance if a current
loop is present ( such as a person touching the metal faucet and reaching into the water to retrieve a hair dryer). The cur-rent
loop due to the water resistance will be in the 100 to 300 mA range, which is considerably less than the 20,000+ mA
needed to trip a 20 amp circuit breaker. Since an appliance will not short out when dropped in a sink or tub, no one should
ever reach into the water to retrieve an appliance accidentally dropped there. The water could be electrified, and a person
touching a grounded object with some other part of the body could receive a serious shock depending on the path the cur-rent
takes through the body. The most important thing to remember is that appliances do not short out when dropped or
submerged in water.
“ Electricity Wants to Go to the Ground”
Sometimes editors of motion films about electrical safety make the statement that “ electricity wants to go to the
ground.” There are even books published about electrical wiring that contain the same statement. As previously stated,
electricity wants to return to the transformer that created it, and the two conductors that were designed to carry it safely
are the preferred route it takes. Whenever current goes to ground or any other ground loop, it is the result of a fault in the
appliance, cords, plugs or other source.
“ It Takes High Voltage to Kill; 120 Volts AC Is Not Dangerous”
Current is the culprit that kills. Voltage determines the form of the injury. Under the right conditions, AC voltage as
low as 60 volts can kill. At higher voltages the body can be severely burned yet the victim could live. Respect all AC
voltages, high or low, as having the potential to kill.
“ Double Insulated Power Tools Are Doubly Safe and Can Be Used in Wet and Damp Locations”
Read the manufacturer’s operating instructions carefully. Double insulated power tools are generally made with materi-al
that is nonconductive. This does give the user protection from electrical faults that occur within the insulated case of
the appliance. However, double insulated power tools can be hazardous if dropped into water. Electrical current can flow
out of the power tool case into the water. Remember that double insulated power tools are not to be used in areas where
they can get wet. If conditions or situations require their use under adverse conditions, use GFCI ( ground fault circuit
interrupter) protection for the employee.
• ampere ( amp)— the unit used to measure current
• milliampere ( milliamp or mA)— 1/ 1,000 of an ampere
• shocking current— electrical current that passes through a part of the body
• You will be hurt more if you can’t let go of a tool giving a shock.
• The longer the shock, the greater the injury.
7
Dangers of Electrical Shock
The severity of injury from electrical shock depends on the amount of electrical current and the length of time the current
passes through the body. For example, 1/ 10 of an ampere ( amp) of electricity going through the body for just 2 seconds is
enough to cause death. The amount of internal current a person can withstand and still be able to control the muscles of the
arm and hand can be less than 10 milliamperes ( milliamps or mA). Currents above 10 mA can paralyze or “ freeze” muscles.
When this “ freezing” happens, a person is no longer able to release a tool, wire or other object. In fact, the electrified object
may be held even more tightly, resulting in longer exposure to the shocking current. For this reason, hand- held tools that give
a shock can be very dangerous. If you can’t let go of the tool, current continues through your body for a longer time, which
can lead to respiratory paralysis ( the muscles that control breathing cannot move). You stop breathing for a period of time.
People have stopped breathing when shocked with currents from voltages as low as 49
volts. Usually, it takes about 30 mA of current to cause respiratory paralysis.
Currents greater than 75 mA cause ventricular fibrillation ( very rapid, ineffective
heartbeat). This condition will cause death within a few minutes unless a special device
called a defibrillator is used to save the victim. Heart paralysis occurs at 4 amps, which
means the heart does not pump at all. Tissue is burned with currents greater than 5 amps. 2
Table 1 shows what usually happens for a range of currents ( lasting one second) at
typical household voltages. Longer exposure times increase the danger to the shock vic-tim.
For example, a current of 100 mA applied for 3 seconds is as dangerous as a cur-rent
of 900 mA applied for a fraction of a second ( 0.03 seconds). The muscle structure
of the person also makes a difference. People with less muscle tissue are typically
affected at lower current levels. Even low voltages can be extremely dangerous because
the degree of injury depends not only on the amount of current but also on the length of
time the body is in contact with the circuit.
LOW VOLTAGE DOES NOT MEAN LOW HAZARD!
Table 1
Effects of Electrical Current* on the Body3
8
Defibrillator in use.
Current Reaction
1 milliamp Just a faint tingle.
5 milliamps Slight shock felt. Disturbing, but not painful. Most people can let go.
However, strong involuntary movements can cause injuries.
6– 25 milliamps ( women)† Painful shock. Muscular control is lost. This is the range where " freezing currents" start.
9– 30 milliamps ( men) It may not be possible to let go.
5– 150 milliamps Extremely painful shock, respiratory arrest ( breathing stops), severe muscle contractions.
Flexor muscles may cause holding on; extensor muscles may cause intense pushing
away. Death is possible.
1,000– 4,300 milliamps Ventricular fibrillation ( heart pumping action not rhythmic) occurs. Muscles contract;
( 1– 4.3 amps) nerve damage occurs. Death is likely.
10,000 milliamps Cardiac arrest and severe burns occur. Death is probable.
( 10 amps)
15,000 milliamps Lowest overcurrent at which a typical fuse or circuit breaker opens a circuit!
( 15 amps)
* Effects are for voltages less than about 600 volts. Higher voltages also cause severe burns.
† Differences in muscle and fat content affect the severity of shock.
____________
2 Lee R. L. [ 1973]. Electrical Safety in Industrial Plants. Am Soc Safety Eng J18( 9): 36- 42.
3 USDOL [ 1997]. Controlling Electrical Hazards. Washington, D. C.: U. S. Department of Labor, Occupational Safety and Health Administration.
Sometimes high voltages lead to additional injuries. High voltages can cause violent muscular contractions. You may
lose your balance and fall, which can cause injury or even death if you fall into machinery that can crush you. High volt-ages
can also cause severe burns ( as seen on photos later in this and other sections).
• High voltages cause additional injuries.
At 600 volts, the current through the body may be as great as 4 amps, causing damage to internal organs such as the
heart. High voltages also produce burns. In addition, internal blood vessels may clot. Nerves in the area of the contact
point may be damaged. Muscle contractions may cause bone fractures from either the contractions themselves or from
falls.
• Higher voltages can cause larger currents and more severe shocks.
A severe shock can cause much more damage to the body than is visible. A person may suffer internal bleeding and
destruction of tissues, nerves, and muscles. Sometimes the hidden injuries caused by electrical shock result in a delayed
death. Shock is often only the beginning of a chain of events. Even if the electrical current is too small to cause injury,
your reaction to the shock may cause you to fall, resulting in bruises, broken bones, or even death.
• Some injuries from electrical shock cannot be seen.
The length of time of the shock greatly affects the amount of injury. If the shock is short in duration, it may only be
painful. A longer shock ( lasting a few seconds) could be fatal if the level of current is high enough to cause the heart to go
into ventricular fibrillation. This is not much current when you realize that a small power drill uses 30 times as much cur-rent
as what will kill. At relatively high currents, death is certain if the shock is long enough. However, if the shock is
short and the heart has not been damaged, a normal heartbeat may resume if contact with the electrical current is eliminat-ed.
( This type of recovery is rare.)
• The greater the current, the greater the shock.
• Severity of shock depends on voltage, amperage, and resistance.
• Resistance— a material’s ability to decrease or stop electrical current.
• Ohm— unit of measurement for electrical resistance.
• Lower resistance causes greater currents.
• Currents across the chest are very dangerous.
The amount of current passing through the body also affects the severity of an electrical shock. Greater voltages pro-duce
greater currents. There is greater danger from higher voltages. Resistance hinders current. The lower the resistance
( or impedance in AC circuits), the greater the current will be. Dry skin may have a resistance of 100,000 ohms or more.
Wet skin may have a resistance of only 1,000 ohms. Wet working conditions or broken skin will drastically reduce resis-tance.
The low resistance of wet skin allows current to pass into the body more easily and give a greater shock. When
more force is applied to the contact point or when the contact area is larger, the resistance is lower, causing stronger
shocks.
The path of the electrical current through the
body affects the severity of the shock. Currents
through the heart or nervous system are most
dangerous. If you contact a live wire with your
head, your nervous system will be damaged.
Contacting a live electrical part with one hand-while
you are grounded at the other side of
your body- will cause electrical current to pass
across your chest, possibly injuring your heart
and lungs.
• NEC— National Electrical Code— a com-prehensive
listing of practices to protect
workers and equipment from electrical
hazards such as fire and electrocution
There have been cases where an arm or leg
is severely burned by high- voltage electrical
9
Power drills use 30 times as much
current as what will kill.
current to the point of coming off, and the victim is not electrocuted. In these cases, the current passes through only a part
of the limb before it goes out of the body and into another conductor. Therefore, the current does not go through the chest
area and may not cause death, even though the victim is severely disfigured. If the current does go through the chest, the
person will almost surely be electrocuted. A large number of serious electrical injuries involve current passing from the
hands to the feet. Such a path involves both the heart and lungs. This type of shock is often fatal.
Summary
The danger from electrical shock depends on
• The amount of the shocking current through the body.
• The duration of the shocking current through the body.
• The path of the shocking current through the body.
10
3
Arc Flash/ NFPA 70E
OSHA revised Subpart S to reflect updated industry practices and technology and to incorporate the 2000 edition of
NFPA 70E, Electrical Safety Requirements for Employee Workplaces, and the 2002 revision of the National Electric
Code ( NEC). NFPA 70E applies to all personnel working on energized equipment greater than 50 volts or equipment that
could produce an arc flash, which means virtually every industry has employees at risk. Under the newly revised Subpart
S— Electrical ( effective Aug. 13, 2007), OSHA as well as NCDOL has not adopted NFPA 70E in its entirety, specifically
excluding some personal protective equipment and clothing requirements in regard to arc flash.
What Is an Arc Flash?
The arc flash is the resulting discharge of energy caused by an arcing fault. An arcing fault is the unintended flow of
current through a medium not intended to carry the current. That just means that the electricity is flowing through some-thing
it should not be; in most cases that result in injury, the medium was the air. The air becomes like a piece of copper,
conducting the electricity; only with the air, you can see the massive discharge of the electrons from the discharging ele-ment.
This is the arc flash. It is lightning on a smaller, yet still deadly, scale.
What causes an arcing fault? The most common causes of an arcing fault are equipment failure, human error ( improper
placement of tools or improper use of equipment), or the conduction of electricity due to foreign particles in the air ( usu-ally
metal shavings). 4
Wearing personal protective equipment is necessary in reducing injury from electrical arc flash accidents, but it is no
substitute for proper safety training, among other best practices in arc safety.
Every day, electrical arc flash accidents injure or kill, but wearing proper personal protective equipment ( PPE) mini-mizes
accident frequency and severity. PPE alone, however, is no substitute for thorough safety training, consistently
following lockout/ tagout procedures, keeping electrical equipment well- maintained, and applying engineering controls.
Burns are not the only risk. A high- amperage arc produces an explosive pressure wave blast that can cause severe fall-related
injuries.
Four- step hazard calculations: First, establish the job’s hazard risk category. Second, determine what clothing and
equipment the hazard risk category requires. Third, identify what arc thermal performance value ( ATPV) rating is neces-sary.
Finally, select personal protective equipment that meets or exceeds the designated ATPV rating.
Arc Flash Clothing
Arc flash clothes are critically important to keep workers safe. Statistics show that five to ten times a day, a worker in
the United States is injured or killed due to an arc flashing accident. 5 The casualties resulting from these accidents are
almost always devastating to the worker involved and to the worker’s family. 5 Perhaps if these workers had been wearing
appropriately rated arc flashing protective equipment, the number of injuries and deaths could have been decreased.
Need for Protective Clothing
What steps can be taken to reduce the risk? NFPA 70E, Standard for Electrical Safety Requirements for Employee
Workplaces, sets standards and regulations for workers working around energized equipment. NFPA 70E defines neces-sary
steps to be taken to properly prevent serious injury in the event of an arc flash accident. NFPA 70E interprets that
workers within the flash protection boundary ( the area where discharged energy is greater than 1.2 cal/ cm2) must be
qualified and wearing thermally resistant and arc flash protective clothing.
11
____________
4 http:// www. lg. com/ about/ newsletter/ June04/ ArcFlash. html
5 http:// www. carolinaseca. org/ pdf/ arcflash. pdf
Arc Flash Clothing Selection
Picking the right type of arc flash protective clothing is easy. First, consult NFPA 70E, 2004 Edition, Table
130.7( C)( 9)( a), to determine to which category of risk a particular activity belongs. Second, consult Table 130.7( C)( 10) to
determine what type of clothing/ equipment is required based on the category of risk determined. Third, consult Table
130.7( C)( 11) to determine the ATPV ( arc thermal performance value) rating needed. Once you have done all this, just go
out and find the protective gear that meets or exceeds this rating. 6 One thing to remember when picking the protective
work wear is to try and ensure that no skin is exposed. Ensure that the pant legs ( if not connected to boots) completely go
down to the boot. Also ensure that the sleeves of the protective work wear go down to the hand, leaving none of the arm
exposed. And lastly, remember that the head is the most vulnerable part of the body. Do not forget to complete the arc
flash protective clothing with suitable head gear of the same ATPV rating as the rest of the work- wear plus high voltage
gloves.
NFPA 70E Table 130.7( C)( 11)
Protective Clothing Characteristics6
When nothing can be done about working within a flash protection boundary, proper arc flash protective clothing needs
to be worn. Workers need to remember that arc flash accidents do not only occur with equipment at high voltage. The
majority of arc flash accidents occur with low ( 120V) and medium voltage ( 480V) equipment. Workers who wear the
proper arc flash protective clothing will significantly reduce the risk of injury or death should an arc flash accident occur.
Summary
Always perform a flash hazard analysis and acquire the appropriate flash retardant clothing ( FRC). Care and
laundering of FRC should be taken cautiously. Employer/ employees must follow safe work practices. Employees must be
adequately trained on electrical safety, with first aid/ CPR training as needed. Whenever possible, lock out all equipment!
12
Hazard Risk Work- wear Description ATPV
Category ( 1/ 2/ 3/ 4) refers to the number of clothing layers Rating cal/ cm2
0 Untreated Cotton ( 1) n/ a
1 FR Shirt and FR Pant ( 1) 5
2 Cotton Undergarments + FR Shirt/ Pant ( 2) 8
3 Cotton Undergarments + FR Shirt/ Pant + FR Coveralls ( 3) 25
4 Cotton Undergarments + FR Shirt/ Pant + Double Layer Switching Coat
and Pant ( 4) 40
* Recommendation is 100 percent cotton.
____________
6 http:// www. labsafety. com/ refinfo/ printpage. htm? page=/ refinfo/ ezfacts/ ezf263. htm
4
Branch Circuit Wiring
Definitions
Discussion of wiring methods must be preceded by an understanding of terms used to define each specific conductor in
a typical 120/ 240 volt AC system. Refer to Figure 4 for an example of most of the following definitions. The National
Electrical Code ( NEC) is used as the reference source.
Ampacity. The current ( in amps) that a conductor can carry continuously under the conditions of use without exceed-ing
its temperature rating. When you find attachment plugs, cords or receptacle face plates that are hot to touch, this may
be an indication that too much of a load ( in amps) is being placed on that branch circuit. If the insulation on the conduc-tors
gets too hot, it can melt and cause arcing, which could start a fire.
Attachment Plug. Describes the device ( plug) that when inserted into the receptacle establishes the electrical connec-tion
between the appliance and branch circuit.
Branch Circuit. The electrical conductors between the final overcurrent device ( the service entrance panel ( SEP) in
Figure 4) protecting the circuit and the receptacle. The wiring from the SEP to the pole mounted transformer is called the
“ service.”
Circuit Breaker. Opens and closes a circuit by nonautomatic means as well as being designed to open automatically at
a predetermined current without causing damage to itself. Be alert to hot spots in circuit breaker panels indicating that the
circuit breaker is being overloaded or that there may be loose connections.
Equipment. A general term for material, fittings, devices, appliances, fixtures, apparatus and the like used as a part of,
or in connection with, an electrical installation. In Figure 4, the SEP and any associated conduit and junction boxes would
be considered equipment.
Feeder. The term given to the circuit conductors between the SEP and the final branch circuit overcurrent device. In
Figure 4 there is no feeder since the SEP is also the final branch circuit overcurrent device.
Ground. A conducting connection ( whether intentional or accidental) between an electrical circuit or equipment and
the earth, or to some conducting body that serves in place of the earth. It is important to remember that a conducting body
can be in the ceiling and that we must not
think of ground as restricted to earth. This
is why maintenance personnel may not
realize that a ground loop exists in the
space above a drop ceiling, due to the elec-trical
conduit and other grounded equip-ment
in that space.
Grounded Conductor. The conductor
in the branch circuit wiring that is inten-tionally
grounded in the SEP. This conduc-tor
is illustrated in Figure 4. From the SEP
to the transformer the same electrical path
is referred to as the neutral. From the final
overcurrent device to the receptacle the
conductor is referred to as the grounded
conductor.
13
120 Volts
120 Volts
Neutral
Grounding Electrode
Conductor on
Premises
Equipment- grounding
Conductor ( green or bare)
Utility
Supply
Service
Ground
Circuit Breaker
Grounded
Conductor
( white or gray)
“ Hot” Conductor
( black or red)
Typical Pole
Transformer
Service
Entrance
Panel
Nickel or
Light- colored
Terminal
Green
Hexagonal- head
Terminal Screw
Brass colored
Terminal
Primary Lines
2.4– 13 kV
Figure 4
Branch Circuit Wiring
Grounding Conductor, Equipment. The conductor used to connect the noncurrent- carrying metal parts of equipment,
raceways and other enclosures to the system grounded conductor at the SEP. The equipment grounding conductor path is
allowed to be a separate conductor ( insulated or noninsulated), or where metal conduit is used, the conduit can be used as
the conductor. There are some exceptions to this such as in hospital operating and intensive care rooms. The equipment
grounding conductor is the human safety conductor of the electrical system in that it bonds all noncurrent- carrying metal
surfaces together and then connects them to ground. By doing this we can prevent a voltage potential difference between
the metal cabinets and enclosures of equipment and machinery. This conductor also acts as a low impedance path ( in the
event of a voltage fault to the equipment case or housing) so that if high fault current is developed, the circuit breaker or
fuse will be activated quickly.
Grounding Electrode Conductor. Used to connect the grounding electrode to the equipment grounding conductor
and/ or to the grounded conductor of the circuit at the service equipment or at the source of a
separately derived system ( see Figure 4).
Overcurrent. Any current in excess of the rated current of equipment or the ampacity of a conductor is considered
overcurrent. This condition may result from an overload, short circuit or a ground fault.
Wiring Methods
The NEC requires the design and installation of electrical wiring to be consistent throughout the facility. To accomplish
this, it is necessary to follow NEC requirements. For 120 volt grounding- type receptacles, the following wiring connec-tions
are required ( see Figure 4).
• The ungrounded or “ hot” conductor ( usually with black or red insulation) is connected to the brass colored terminal
screw. This terminal and the metal tension springs form the small slot receiver for any appliance attachment plug.
An easy way to remember the color coding is to remember “ black to brass” or the initials “ B & B.”
• The “ grounded conductor” insulation is generally colored white ( or gray) and should be fastened to the silver or light
colored terminal. This terminal and the metal tension springs form the large slot for a polarized attachment plug. An
easy way to remember this connection is to think “ white to light.”
• The equipment grounding conductor path can be a conductor, or where metal conduit is used, the conduit can be sub-stituted
for the conductor. If the latter is used, you must monitor the condition of the conduit system to ensure that it
is not damaged or broken. Any “ open” in the conduit system will eliminate the equipment grounding conductor
path. Additionally, the condition of the receptacles must be monitored to ensure that they are securely fastened to
the receptacle boxes. When a third wire is run to the receptacle either in a conduit or as a part of a nonmetallic
sheathed cable assembly, the conductor must be connected to the green colored terminal on the receptacle.
These wiring methods must be used to ensure that the facility is correctly wired. Circuit testing methods will be dis-cussed
in Part 5. In older homes, knob and tube or other two- wire systems may be present. The NEC requires that ground-ing
type receptacles be used as replacements for existing nongrounding types and be connected to a grounding conductor.
An exception is that where a grounding means does not exist in the enclosure, either a nongrounding or a GFCI- type
receptacle must be used. A grounding conductor must not be connected from the GFCI receptacle to any outlet supplied
from the GFCI receptacle. The exception further allows nongrounding type receptacles to be replaced with the grounding
type where supplied through a GFCI receptacle.
Plug and Receptacle Configurations
Attachment plugs are devices that are fastened to the end of a cord so that electrical contact can be made between the
conductor in the equipment cord and the conductors in the receptacle. The plugs and receptacles are designed for different
voltages and currents, so that only matching plugs will fit into the correct receptacle. In this way, a piece of equipment
rated for one voltage and/ or current combination cannot be plugged into a power system that is of a different voltage or
current capacity.
The polarized three- prong plug is designed with the equipment grounding prong slightly longer than the two parallel
blades. This provides equipment grounding before the equipment is energized. Conversely, when the plug is removed
from the receptacle, the equipment grounding prong is the last to leave, ensuring a grounded case until power is removed.
The parallel line blades maybe the same width on some appliances since the three- prong plug can only be inserted in one
14
way. A serious problem results whenever a person breaks or cuts off the grounding prong. This not only voids the safety
of the equipment grounding conductor but allows the attachment plug to be plugged in with the correct polarity or with
the wrong polarity.
Figure 5 illustrates some of the
National Electrical Manufacturers
Association ( NEMA) standard plug and
receptacle connector blade configura-tions.
Each configuration has been devel-oped
to standardize the use of plugs and
receptacles for different voltages,
amperes, and phases from 115 through
600 volts and from 10 through 60 amps,
and for single- and three- phase systems.
You should be alert to jury- rigged
adaptors used to match, as an example, a
50 amp attachment plug to a 20 amp
receptacle configuration. Using these
adaptors poses the danger of mixing volt-age
and current ratings and causing fire
and/ or shock hazards to personnel using
equipment. Equipment attachment plugs
and receptacles should match in voltage
and current ratings to provide safe power
to meet the equipment ratings. Also the
attachment plug cord clamps must be
secured to the cord to prevent any strain
or tension from being transmitted to the ter-minals
and connections inside the plug.
Understanding Reverse Polarity
The NEC recognizes the problem of reverse polarity. It states that no grounded conductor may be attached to any ter-minal
or lead so as to reverse the designated polarity. Many individuals experienced with electrical wiring and appliances
think that reverse polarity is not hazardous. A few example situations should heighten your awareness of the potential
shock hazard from reverse polarity.
An example of one hazardous situation
would be an electric hand lamp. Figure 6
illustrates a hand lamp improperly wired and
powered.
When the switch is turned off, the shell of
the lamp socket is energized. If a person acci-dentally
touched the shell ( while changing a
bulb) with one hand and encountered a
ground loop back to the transformer, a shock
could result. If the lamp had no switch and
was plugged in as shown, the lamp shell
would be energized when the plug was insert-ed
into the receptacle. Many two- prong plugs
have blades that are the same size, and the
right or wrong polarity is just a matter of
chance. If the plug is reversed ( Figure 7), the
voltage is applied to the bulb center terminal
15
Only
5- 15R 5- 15P
5- 20R 5- 20P
10- 50R 10- 50P
10- 30R 10- 30P
6- 30R 6- 30P
125/ 250- Volt, 30- Ampere
Receptacle and Plug
250- Volt, 30- Ampere
Receptacle and Plug
125/ 250- Volt, 50- Ampere
Receptacle and Plug
20- Ampere Plug,
125- Volt Receptacles
and Plugs
Either
15- Ampere Plug
15 Ampere
20 Ampere
W
X Y
W
Y X
W
Y X
W
X Y
W
G
W
G
W
G
W
G
G G
Figure 5
Plug and Receptacle Configurations
Switch
OFF Position
Bulb
Metal Guard
Ground Potential
Equipment Plug
Receptacle
“ Hot”
“ Hot”
SEP
120 Volt
Transformer
Figure 6
Hand Lamp— Reverse Polarity
and the shell is at ground potential. Contact
with the shell and ground would not create a
shock hazard in this situation. In this example
the hand lamp is wired correctly.
Another example is provided by electric
hair dryers or other plastic- covered electrical
appliances. Figure 8 illustrates a hair dryer
properly plugged into a receptacle with the
correct polarity.
You will notice that the switch is single
pole- single throw ( SPST). When the appli-ance
is plugged in with the switch in the hot
or 120 volt leg, the voltage stops at the switch
when it is in the off position. If the hair dryer
were accidentally dropped into water, current could flow out of the plastic housing, using the water as the conducting
medium. The water does not short out the appliance since the exposed surface area of the hot wire connection to the
switch terminal offers such a high resis-tance.
This limits the current flow to less
than 1 mA ( correct polarity). The fault
current is not sufficient to trip a 20- amp
circuit breaker. To trip the circuit break-er,
there would have to be a line- to- line
short that would cause an excess of 20
amps ( 20,000 mA). Should a person try
to retrieve the appliance from the water
while it is still plugged into the outlet?
In this configuration, the fault current
would be extremely low ( unless the
switch were in the on position). Since
you have no way to tell if the polarity is
correct, don’t take chances. NEVER
REACH INTO WATER TO RETRIEVE
AN APPLIANCE. Always unplug the appliance first, then retrieve the appliance and dry it out.
If the appliance is plugged in as shown in Figure 9, when the switch is off, voltage will be present throughout all the
internal wiring of the appliance. Now if it is dropped into water, the drastically increased “ live” surface area will allow a
drastic increase in the available electric current ( I = E/ R). A person who accidentally tries to retrieve the dryer would be in
a hazardous position because the voltage in the water could cause current to flow through the body ( if another part of the
body contacts a ground loop). This illus-trates
the concept that reverse polarity is a
problem whenever appliances are used
with plastic housings in areas near sinks,
or where the appliance is exposed to rain
or water. Remember, most motorized
appliances have air passages for cooling.
Wherever air can go, so can moisture and
water.
If the appliance had a double pole- dou-ble
throw switch ( DPDT), it would make
no difference how the plug was positioned
in the outlet. The hazard would be mini-mized
since the energized contact surface
would be extremely small. If the appliance
16
Switch
OFF Position
Bulb
Metal Guard
Ground Potential
Equipment Plug
Receptacle
“ Hot”
“ Hot”
“ Hot”
SEP
120 Volt
Transformer
Figure 7
Hand Lamp— Correct Polarity
Service
Entry
Panel
( SEP)
120 Volt
Transformer
Plug
Receptacle
“ Hot”
“ Hot”
Hair Dryer
Switch
Figure 8
Hair Dryer— Correct Polarity
Service
Entry
Panel
( SEP)
120 Volt
Transformer
Plug
Receptacle
“ Hot”
“ Hot”
Hair Dryer
Switch
Figure 9
Hair Dryer— Reverse Polarity
were dropped into water, a high resistance contact in the water and a resulting low available fault current ( I = E/ R) would
result. Later, we will see how GFCIs can be used to protect against shock hazards when using appliances with noncon-ductive
housings around water.
Remember that any electric appliance dropped in water or accidentally exposed to moisture should be considered as ener-gized.
The electric power must be safely removed from the appliance before it is retrieved or picked up. You never know if
the appliance is plugged in with the right polarity without test equipment. Do not take chances. Remove the power first.
Grounding Concepts
Grounding falls into two safety categories. It is important to distinguish between “ system grounding” and “ equipment
grounding.” Figure 10 illustrates these two grounding components. The difference between these two terms is that system
grounding actually connects one of the current carrying conductors from the supply transformer to ground. Equipment
grounding connects or bonds all of the noncurrent- carrying metal surfaces together and then is connected to ground.
System grounding ( Figure 10) at the transformer provides a grounding point for the power company surge and light-ning
protection devices. In conjunction with the system grounding at the SEP, the voltage across system components is
limited to a safe value should they be subject-ed
to lightning or high voltage surges. The
system grounding at the SEP also helps to
limit high voltages from entering the electri-cal
system beyond the SEP. It is important to
check all of the connections both indoors and
outdoors since many times they are exposed
to moisture, chemicals and physical damage.
Equipment grounding does two things.
First, it bonds all noncurrent- carrying metal
surfaces together so that there will be no
potential difference between them. Second, it
provides a path for current to flow under
ground fault conditions. The equipment
grounding path must have low impedance to
ensure rapid operation of the circuit overcur-rent
device should a “ hot” to ground fault
occur.
Figure 11 depicts some common equip-ment
faults that can occur. A problem with
the equipment grounding system is that under
normal conditions it is not a current- carrying
conductor and a fault would not be readily
detected. Necessary visual inspection will not
provide an operational verification. In Figure
11 we can test the condition of the equipment
grounding conductor using test procedures in
this guide. To test the quality of the branch
circuit equipment grounding system, a special
tester called a ground loop impedance tester
is needed. It is generally recommended that
an impedance of 0.5 ohms be achieved in the
equipment grounding conductor path. In case
of a “ hot” to ground fault, the fault current
would quickly rise to a value ( I = E/ R =
120/ 0.5 = 240 amps) necessary to trip a 20
amp fuse or breaker.
17
Equipment Grounding
Equipment
Grounding
Conductor
Electrical Symbol
For Ground
Service
Entrance
Grounded
Conductor or
Neutral
Transformer
Secondary
Primary
Most Metallic Raceways, Cable Sheaths
and Cable Armor That Are Continuous
and Utilize Proper Fittings May Serve as
the Equipment Grounding Conductor. A
Separate Grounding Conductor is Needed
When Plastic Conduit, Non- metallic
Sheathed Cable or Other Wiring Methods
Are Used That Are Not Approved as
Grounding Methods.
Figure 10
System and Equipment Grounding
? ? Fault
White
Black
Receptacle Ground
Fault
Tool or
Appliance
Grounding Prong
Missing
Ground Fault
Current
Wire or
Conduit
Service
Entrance
Source
Figure 11
Equipment Grounding Faults
If there is an open or break in the conduit system, as shown in Figure 11, a fault in a power tool with a good grounding
prong would allow the voltage to be placed directly on the ungrounded conduit. This would create a serious shock hazard
to anyone touching the conduit with one hand while touching a ground loop with the other. The missing ground prong
( with a good conduit or equipment grounding path) would create a serious shock hazard to the person if the ground loop
through the feet was low resistance ( e. g., wet earth or concrete and wet shoes). It is important to emphasize the need for
low impedance on the equipment grounding loop. If the ground fault current loop in Figure 11 were 25 ohms and the
body resistance of the person were 850 ohms, then the 25 ohm ground loop resistance would be too high to cause enough
circuit breaker current to trip it open. In this case, the person would then receive multiples of current considered deadly
( 141 mA in this case) through the body, causing death in most instances. Remember that standard circuit breakers are for
equipment and fire protection, not people protection. However, ground fault circuit interrupter ( GFCI) circuit breakers are
specifically designed for people protection.
Burns Caused by Electricity
The most common shock- related, nonfatal injury is a burn. Burns caused by electricity may be of three types: electrical
burns, arc burns and thermal contact burns. Electrical burns can result when a person touches electrical wiring or
equipment that is used or maintained improperly. Typically such burns occur on the hands. Electrical burns are one of the
most serious injuries you can receive. They need to be given immediate attention. Additionally, clothing may catch fire and
a thermal burn may result from the heat of the fire.
• Electrical shocks cause burns.
Arc- blasts occur when powerful, high- amperage currents arc through the air. Arcing is the luminous electrical dis-charge
that occurs when high voltages exist across a gap between conductors and current travels through the air. This situ-ation
is often caused by equipment failure due to abuse or fatigue. Temperatures as high as 35,000 F have been reached in
arc- blasts.
• arc- blast— explosive release of molten material from equipment caused by high- amperage arcs
• arcing— the luminous electrical discharge ( bright, electrical sparking) through the air that occurs when high volt-ages
exist across a gap between conductors
There are three primary hazards associated with an arc- blast.
1. Arcing gives off thermal radiation ( heat) and intense light, which can cause burns. Several factors affect the degree
of injury, including skin color, area of skin exposed and type of clothing worn. Proper clothing, work distances and
overcurrent protection can reduce the risk of such a burn.
2. A high- voltage arc can produce a considerable pressure wave blast. A person 2 feet away from a 25,000- amp arc
feels a force of about 480 pounds on the front of the body. In addition, such an explosion can cause serious ear
damage and memory loss due to concussion. Sometimes the pressure wave throws the victim away from the arc-blast.
While this may reduce further exposure to the thermal energy, serious physical injury may result. The pres-sure
wave can propel large objects over great distances. In some cases, the pressure wave has enough force to snap
off the heads of steel bolts and knock over walls.
3. A high- voltage arc can also cause many of the copper and aluminum components in electrical equipment to melt.
These droplets of molten metal can be blasted great distances by the pressure wave. Although these droplets harden
rapidly, they can still be hot enough to cause serious burns or cause ordinary clothing to catch fire, even if you are
10 feet or more away.
18
Electrical Fires
Electricity is one of the most common causes of fires and thermal burns in homes and workplaces. Defective or mis-used
electrical equipment is a major cause of electrical fires. If there is a small electrical fire, be sure to use only a Class
C or multi- purpose ( ABC) fire extinguisher, or you might make the problem worse. All fire extinguishers are marked with
letter( s) that tell you the kinds of fires they can put out. Some extinguishers contain symbols, too. The letters and symbols
are explained below ( including suggestions on how to remember them).
A( think: Ashes) = paper, wood, etc.
B( think: Barrel) = flammable liquids
C( think: Circuits) = electrical fires
Here are a couple of fire extinguishers at a worksite. Can you tell what types of
fires they will put out?
19
Five technicians were performing preventive maintenance on the electrical system of a railroad maintenance facility.
One of the technicians was assigned to clean the lower compartment of an electrical cabinet using cleaning fluid in
an aerosol can. He began to clean the upper compartment as well. The upper compartment was filled with live cir-cuitry.
When the cleaning spray contacted the live circuitry, a conductive path for the current was created. The cur-rent
passed through the stream of fluid, into the technician’s arm and across his chest. The current caused a loud
explosion. Co- workers found the victim with his clothes on fire. One worker put out the fire with an extinguisher, and
another pulled the victim away from the compartment with a plastic vacuum cleaner hose. The paramedics
responded in 5 minutes. Although the victim survived the shock, he died 24 hours later of burns.
This death could have been prevented if the following precautions had been taken:
• Before doing any electrical work, de- energize all circuits and equipment, perform lockout/ tagout, and test circuits
and equipment to make sure they are de- energized.
• The company should have trained the workers to perform their jobs safely.
• Proper personal protective equipment ( PPE) should always be used.
• Never use aerosol spray cans around high- voltage equipment.
This extinguisher can only be used on Class B
and Class C fires.
This extinguisher can only be used on
Class A and Class C fires.
Thermal burns may result if an explosion occurs when electricity ignites an explosive mixture of material in the air.
This ignition can result from the buildup of combustible vapors, gases or dusts. OSHA standards, the NEC and other safe-ty
standards give precise safety requirements for the operation of electrical systems and equipment in such dangerous
areas. Ignition can also be caused by overheated conductors or equipment, or by normal arcing at switch contacts or in
circuit breakers.
Summary
Burns are the most common injury caused by electricity. The three types of burns are electrical burns, arc burns and
thermal contact burns.
20
Note: However, do not try to put out fires unless you have
received proper training. If you are not trained, the best thing
you can do is evacuate the area and call for help.
5
Branch Circuit and Equipment Testing
Testing Branch Circuit Wiring
Branch circuit receptacles should be tested periodically. The frequency of testing should be established on the basis of
outlet usage. In shop areas, quarterly testing may be necessary. Office areas may only need annual testing. A preventive
maintenance program should be established. It is not unusual to find outlets as old as the facility. For some reason, a pop-ular
belief exists that outlets never wear out. This is false. For example, outlets take severe abuse from employees discon-necting
the plug from the outlet by yanking on the cord. This can put severe strain on the contacts inside the outlet as well
as on the plastic face.
The electrical receptacle is a critical electrical system component. It must provide a secure mechanical connection for
the appliance plug so that there is a continuous electrical circuit for each of the prongs. Receptacles must be wired correct-ly,
or serious injuries can result from their use. For this reason, the following two- step testing procedure is recommended.
Receptacle Testing
Step 1
Plug in a three- prong receptacle circuit tester and note the combination of the
indicator lights ( see Figure 12). The tester checks the receptacle for the proper con-nection
of the grounding conductor, wiring polarity and other combinations of
wiring errors. If the tester checks the receptacle as OK, proceed to the next step. If
the tester indicates a wiring problem, have it corrected as soon as possible. Retest
after the problem is corrected.
Step 2
After the outlet has been found to be
wired ( electrically) correct, the receptacle
contact tension test must be made. A typical tension tester is shown in Figure 13.
The tension should be 8 ounces or more. If it is less than 8 ounces, have it
replaced. The first receptacle function that loses its contact tension is usually the
grounding contact circuit. The plug grounding prong is the last part to leave the
receptacle. Because of the leverage ( due to its length) it wears out the tension of
the receptacle contacts quicker than the parallel blade contacts. Since this is the
human safety portion of the grounding system, it is very important that this contact
tension be proper. In determining the frequency of testing, the interval must be
based on receptacle usage. All electrical maintenance personnel should be
equipped with receptacle circuit and tension testers. Other maintenance employees could also be equipped with testers
and taught how to use them. In this manner, the receptacles can be tested before maintenance workers use them. Home
receptacles should also be tested.
Testing Extension Cords
New extension cords should be tested before being put into service. Many inspectors have found that new extension
cords have open ground or reverse polarity. Do not assume that a new extension cord is correct. Test it.
Extension cord testing and maintenance are extremely important. The extension cord takes the electrical energy from a
fixed outlet or source and provides this energy at a remote location. The extension cord must be wired correctly, or it can
become the critical fault path.
Testing of extension cords new and used should use the same two steps as used in electrical outlet testing. These two
steps are:
21
Figure 12
Receptacle Tester
Figure 13
Receptacle Tension Tester
Step 1
Plug the extension cord into an electrical outlet that has successfully passed the outlet testing procedure. Plug in any
three- prong receptacle circuit tester into the extension receptacle and note the combination of indicator lights. If the tester
checks the extension cord as OK, proceed to the next step. If the tester indicates a faulty condition, repair or replace the
cord. Once the extension cord is correctly repaired and passes the three- prong circuit tester test, proceed to step 2.
Step 2
Plug a reliable tension tester into the receptacle end. The parallel receptacle contact tension and the grounding contact
tension should check out at 8 ounces or more. If the tension is less than 8 ounces, the receptacle end of the extension cord
must be repaired. As with fixed electrical outlets, the receptacle end of an extension cord loses its grounding contact ten-sion
first. This path is the critical human protection path and must be both electrically and mechanically in good condition.
Testing Plug- and Cord- Connected Equipment
The last element of the systems test is the testing of plug- and cord- connected equipment. The electrical inspector
should be on the alert for jury- rigged repairs made on power tools and appliances. Many times a visual inspection will
disclose three- prong plugs with the grounding prong broken or cut off. In such instances, the grounding path to the equip-ment
case has been destroyed. ( The plug can now also be plugged into the outlet in the reverse polarity configuration.)
Double insulated equipment generally has a nonconductive case and will not be tested using the procedures discussed
below. Some manufacturers that have listed double insulation ratings may also provide the three- prong plug to ground
any exposed noncurrent- carrying metal parts. In these cases, the grounding path continuity can be tested.
A common error from a maintenance standpoint is the installation of a three- prong plug on a two- conductor cord to the
appliance. Obviously, there will be no grounding path if there are only two conductors in the cord. Some hospital grade
plugs have transparent bases that allow visual inspection of the electrical connection to each prong. Even in that situation,
you should still perform an electrical continuity test.
Maintenance shops may have commercial power tool testing equipment. Many power tool testers require the availabili-ty
of electric power. The ohmmeter can be used in the field and in locations where electric power is not available or is not
easily obtained. Plug- and cord- connected equipment tests are made on de- energized equipment. Testing of de- energized
equipment in wet and damp locations can also be done safely.
The plug- and cord- connected equipment test using a self- contained battery- powered ohmmeter is simple and straight-forward.
The two- step testing sequence that can be performed on three- prong plug grounded equipment follows:
Step 1— Continuity Test— Ground Pin to Case Test
Set the ohmmeter selector switch to the lowest scale
( such as R X 1). Zero the meter by touching the two test
probes together and adjusting the meter indicator to zero.
Place one test lead ( tester probe) on the grounding pin of
the de- energized equipment as shown in Figure 14. While
holding that test lead steady, take the other test lead and
make contact with an unpainted surface on the metal case
of the appliance. You should get a reading of less than 1
ohm. If the grounding path is open, the meter will indicate
infinity. If there is no continuity, then the appliance must
be tagged out and removed from service. If the appliance
grounding path is OK, proceed to step 2.
22
White
MOTOR
Green
Silver
Brass
Black
Metal Case
Tester Probe
Tester Probe
SPST
Switch
R X 1
Scale
Reading 1 ohm
or less
Figure 14
Continuity Test
Step 2— Leakage Test— Appliance Leakage Test
Place the ohmmeter selector switch on the highest ohm test position ( such as R X 1,000). Set the meter at zero. Place
one test lead on an unpainted surface of the appliance case ( see Figure 15), then place the other test lead on one of the
plug’s parallel blades.
Observe the reading. The ideal is close to infinity. ( If
a reading of less than 1 meg- ohm is noted, return the
appliance to maintenance for further testing.) With one
test lead still on the case, place the other test lead on
the remaining parallel blade and note the ohmmeter
reading. A reading approaching infinity is required.
( Again, anything less than 1 meg- ohm should be
checked by a maintenance shop.)
23
White
MOTOR
Green
Silver
1.
2.
Brass
Black
Metal Case
Tester Probe
Tester Probe
SPST
Switch
R X 1,000
Scale
Reading
8
Figure 15
Leakage Test
6
Voltage Detector Testing
Operation
When you are conducting an electrical inspection, a voltage detector should be used in conjunction with the circuit
tester, ohmmeter and tension tester. Several types of these devices are inexpensive and commercially available.
These testers are battery powered and are constructed of nonconduct-ing
plastic. Lightweight and self- contained, these testers make an ideal
inspection tool.
The voltage detector works like a radio receiver in that it can receive
or detect the 60 hertz electromagnetic signal from the voltage waveform
surrounding an ungrounded (“ hot”) conductor. Figure 17 illustrates the
detector being used to detect the “ hot” conductor in a cord connected to
a portable hand lamp. When the front of the detector is placed near an
energized “ hot” or ungrounded conductor, the tester will provide an
audible as well as a visual warning. In Figure 17 if the plug were dis-connected
from the receptacle, the detector would not sound an alarm
since there would not be any voltage waveform present.
Typical Voltage Detector Uses
The detector can also be used to test for properly
grounded equipment. When the tester is positioned on a
properly grounded power tool ( e. g., the electric drill in
Figure 18), the tester will not sound a warning. The rea-son
is that the electromagnetic field is shielded from the
detector so that no signal is picked up. If the grounding
prong had been removed and the drill were not ground-ed,
the electromagnetic waveform would radiate from
the drill and the detector would receive the signal and
give off an alarm.
As illustrated in Figure 18, the detector can be used
to test for many things during an inspection.
Receptacles can be checked for proper AC polarity.
Circuit breakers can be checked to determine if they are
on or off. All fixed equipment can be checked for prop-er
grounding. If the detector gives a warning indication
on any equipment enclosed by metal, perform further
testing with a volt- ohmmeter. Ungrounded equipment
can be ungrounded or, in addition, there may be a fault
to the enclosure making it “ hot” with respect to ground.
An experienced inspector should use the voltmeter to
determine if there is voltage on the enclosure. The volt-age
detector should be used as an indicating tester and
qualitative testing should be accomplished with other
testing devices such as a volt- ohmmeter. The use of the
detector can speed up the inspection process by allow-ing
you to check equipment grounding quickly and
safely.
24
Switch
Audible Alarm
Test Marker
“ Red” Warning Light
“ Green” Power “ On” Light
Figure 16
Voltage Detector
SEP
“ Hot”
“ Hot”
120 Volt
Receptacle
Transformer
Figure 17
Testing for “ Hot” Conductor
Check Fixed
Equipment
Grounding
Easily Check
Power Tools for
Proper Grounding
Safely and Quickly
Check for
Voltage
at AC Outlets
Quickly
Check
Circuit
Breakers
Figure 18
Typical Voltage Detector Uses
Voltage Versus Detection Distance
Another unique feature of the voltage detector shown in Figure 18 is that it is voltage sensitive. Figure 19 lists dis-tances
versus voltage at which the detector “ red” light will turn on. As an example, a conductor energized with 120 volts
AC can be expected to be detected from 0 to 1 inch from the conductor.
The transformer secondary wiring on a furnace
automatic ignition system rated at 10,000 volts
can be expected to be detected from 6 to 7 feet
away. You can use this feature to assist in making
judgments regarding the degree of hazard and
urgency for obtaining corrective action.
When you are using the voltage detector, you
must understand how it operates to interpret its
warning light properly. If you apply the tester to
an energized power tool listed as double insulat-ed,
you will see the red warning light turn on. In
this situation, the voltage waveform is detected
because there is no metal enclosure to shield the
waveform. The same would occur if you were to
test in any of the typewriters used in offices. This does not mean that the plastic or nonmetallic enclosed equipment is
unsafe, only that it is energized and not a grounding type piece of equipment. The examples and explanation of the opera-tional
features should be understood fully. Using the detector provides the opportunity for finding many electrical hazards
that others may have overlooked.
25
To Determine Approximate Voltage:
Slowly approach the circuitry being tested with the front sensor of the
unit and observe the distance at which the red LED light glows along
with the “ beep” sound. Use the chart below to determine the approxi-mate
voltage in the circuitry.
VOLTAGE ( V) 100 200 600 1K 5K 9K
DISTANCE ( inch) 0– 1 1– 2 3– 5 15 5 ft 6– 7 ft up
These figures may vary due to conditions governing the testing, i. e.,
static created by your standing on grounded material, carpets, etc.
Figure 19
Voltage vs. Detection Distance
7
Ground Fault Circuit Interrupters
Operational Theory
The ground fault circuit interrupter ( GFCI) is a fast- acting device that monitors the current flow to a protected load. The
GFCI can sense any leakage of current when current returns to the supply transformer by any electrical loop other than
through the white ( grounded conductor) and the black ( hot) conductors. When any “ leakage current” of 5 mA or more is
sensed, the GFCI, in a fraction of a second, shuts off the current on both the “ hot” and grounded conductors, thereby interrupt-ing
the fault current to the appliance and the fault loop.
This is illustrated in Figure 20. As long as I1 is equal to I2 ( normal appliance operation with no ground fault leakage),
the GFCI switching system remains closed. If a fault occurs between the metal case of an appliance and the “ hot” conduc-tor,
fault current I3 will cause an imbalance ( 5 mA or greater for human protection) allowing the GFCI switching system
to open ( as illustrated) and the removal of power from both the white and the “ hot” conductors.
Another type of ground fault can occur
when a person comes in contact with a “ hot”
conductor directly or touches an appliance
with no ( or a faulty) equipment grounding
conductor. In this case I4 represents the fault
current loop back to the transformer. This
type of ground fault is generally the type that
individuals are exposed to.
The GFCI is intended to protect people. It
de- energizes a circuit, or portion thereof, in
approximately 1/ 40 of a second when the
ground fault current exceeds 5 mA. The
GFCI should not be confused with ground
fault protection ( GFP) devices that protect
equipment from damaging line- to- ground
fault currents. Protection provided by GFCIs
is independent of the condition of the equip-ment
grounding conductor. The GFCI can protect personnel even when the equipment grounding conductor is accidental-ly
damaged and rendered inoperative.
The NEC requires that grounding type receptacles be used as replacements for existing nongrounding types. Where a
grounding means does not exist in the receptacle enclosure, the NEC allows either a nongrounding or GFCI receptacle.
Nongrounding type receptacles are permitted to be replaced with grounding type receptacles when powered through a
GFCI.
Remember that a fuse or circuit breaker cannot provide “ hot” to ground loop protection at the 5 mA level. The fuse or
circuit breaker is designed to trip or open the circuit if a line to line or line to ground fault occurs that exceeds the circuit
protection device rating. For a 15 amp circuit breaker, a short in excess of 15 amps or 15,000 mA would be required. The
GFCI will trip if 0.005 amps or 5 mA start to flow through a ground fault in a circuit it is protecting. This small amount
( 5 mA), flowing for the extremely short time required to trip the GFCI, will not electrocute a person but will shock the
person in the magnitude previously noted.
Typical Types of GFCIs
GFCIs are available in several different types. Figure 21 illustrates three of the types available ( portable, circuit break-er
and receptacle).
26
“ Hot”
Supply
I 1
I 2
I 3
I 4
N. C.
Protected
Load
Equipment
Grounding
Conductor
Sensing and
Test Circuit
External
Ground
Fault
Internal
Equipment
Fault
Grounded
Conductor
Differential
Transformer GFCI Protection Device
Figure 20
Circuit Diagram for a GFCI
Circuit breakers can be purchased with the GFCI protection built in. These combination types have the advantage of
being secured in the circuit breaker panel to prevent unauthorized persons from having access to the GFCI. The receptacle
type GFCI is the most convenient in that it can be tested and/ or reset at the location where it is used. It can also be
installed so that if it is closest to the circuit breaker panel, it will provide GFCI protection to all receptacles on the load
side of the GFCI. The portable type can be carried in a maintenance person’s tool box for instant use at a work location
where the available AC power is not GFCI protected. Other versions of GFCIs are available including multiple outlet
boxes for use by several power tools and for outdoor applications. A new type is available that can be fastened to a per-son’s
belt and carried to the job location.
The difference between a regular electrical recep-tacle
and a GFCI receptacle is the presence of the
TEST and RESET buttons. The user should follow
the manufacturer’s instructions regarding the testing
of the GFCI. Some instructions recommend pushing
the TEST button and resetting it monthly. Defective
units should be replaced immediately.
Be aware that a GFCI will not protect the user
from line ( hot) to line ( grounded conductor) electri-cal
contact. If a person were standing on a surface
insulated from ground ( e. g., a dry, insulated floor
mat) while holding a faulty appliance with a “ hot”
case in one hand, then reached with the other hand to
unplug an appliance plug that had an exposed
grounded ( white) conductor, a line to line contact
would occur. The GFCI would not protect the person
( since there is no ground loop). To prevent this type
of accident from occurring, it is very important to have an ensured equipment grounding conductor inspection program in
addition to a GFCI program. GFCIs do not replace an ongoing electrical equipment inspection program. GFCIs
should be considered as additional protection against the most common form of electrical shock and electrocution: the
line (“ hot”) to ground fault.
GFCI Uses
GFCIs should be used in dairies, breweries, canneries, steam plants, construction sites, and inside metal tanks and boil-ers.
They should be used where workers are exposed to humid or wet conditions and may come in contact with ground or
grounded equipment. They should be used in any work environment that is or can become wet and in other areas that are
highly grounded.
Nuisance GFCI Tripping
When GFCIs are used in construction activities, the GFCI should be located as close as possible to the electrical equip-ment
it protects. Excessive lengths of electrical temporary wiring or long extension cords can cause ground fault leakage
current to flow by capacitive and inductive coupling. The combined leakage current can exceed 5 mA causing the GFCI
to trip—“ nuisance tripping.” GFCIs are now available that can be fastened to your belt. You can plug the extension cord
into the GFCI and use the protected duplex to power lighting and power tools. This is the ideal GFCI for construction and
maintenance workers. It also allows the GFCI to be near the user location, thereby reducing nuisance trips.
Other nuisance tripping may be caused by:
• Outdoor GFCIs not protected from rain or water
• Bad electrical equipment with case to hot conductor fault
• Too many power tools on one GFCI branch
• Resistive heaters
• Coiled extension cords ( long lengths)
• Poorly installed GFCI
27
Receptacle
Type
Circuit Breaker
Type
Portable Type
Figure 21
Typical Types of GFCIs
• Electromagnetic induced current near high voltage lines
• Portable GFCI plugged into a GFCI protected branch circuit
Remember that a GFCI does not prevent shock. It limits the duration of the shock so that the heart does not go
into ventricular fibrillation. The shock lasts about 1/ 40 of a second and can be intense enough to knock a person off a
ladder or otherwise cause an accidental injury. Also, remember that the GFCI does not protect against line- to- line
shock. Be sure to check insulation and connections on power tools each time before use.
Overview of the Safety Model
What Must Be Done to Be Safe?
Use the three- stage safety model: recognize, evaluate and control hazards. To be safe, you must think about your job
and plan for hazards. To avoid injury or death, you must understand and recognize hazards. You need to evaluate the situ-ation
you are in and assess your risks. You need to control hazards by creating a safe work environment, by using safe
work practices, and by reporting hazards to a supervisor, trainer or appropriate person( s).
• Use the safety model to recognize, evaluate and control hazards.
• Identify electrical hazards.
• Don’t listen to reckless, dangerous people.
If you do not recognize, evaluate and control hazards, you may be injured or killed by the
electricity itself, electrical fires or falls. If you use the safety model to recognize, evaluate, and
control hazards, you are much safer.
Stage 1: Recognize Hazards
The first part of the safety model is recognizing the hazards around you. Only then can you
avoid or control the hazards. It is best to discuss and plan hazard recognition tasks with your
co- workers. Sometimes we take risks ourselves, but when we are responsible for others, we are
more careful. Sometimes others see hazards that we overlook. Of course, it is possible to be
talked out of our concerns by someone who is reckless or dangerous. Don’t take a chance.
Careful planning of safety procedures reduces the risk of injury. Decisions to lock out and tag out circuits and equipment
need to be made during this part of the safety model. Plans for action must be made now.
Stage 2: Evaluate Hazards
When evaluating hazards, it is best to identify all possible hazards first, then evaluate the
risk of injury from each hazard. Do not assume the risk is low until you evaluate the hazard.
It is dangerous to overlook hazards. Jobsites are especially dangerous because they are
always changing. Many people are working at different tasks. Jobsites are frequently
exposed to bad weather. A reasonable place to work on a bright, sunny day might be very
hazardous in the rain. The risks in your work environment need to be evaluated all the time.
Then, whatever hazards are present need to be controlled.
• Evaluate your risk.
Stage 3: Control Hazards
Once electrical hazards have been recognized and evaluated, they must be controlled. You control electrical hazards in
two main ways: ( 1) create a safe work environment and ( 2) use safe work practices. Controlling electrical hazards ( as
well as other hazards) reduces the risk of injury or death.
28
Report hazards to your
supervisor, trainer or
other personnel as
appropriate.
OSHA regulations, the NEC and the National Electrical Safety Code ( NESC) provide a wide range of safety informa-tion.
Although these sources may be difficult to read and understand at first, with practice they can become very useful
tools to help you recognize unsafe conditions and practices. Knowledge of OSHA standards is an important part of
training for electrical apprentices. See the appendix for a list of relevant standards.
Always lock out and tag out
circuits.
• Take steps to control hazards:
• Create a safe workplace.
• Work safely.
Safety Model Stage 1— Recognizing
Hazards
How Do You Recognize Hazards?
The first step toward protecting yourself is
recognizing the many hazards you face on the
job. To do this, you must know which situa-tions
can place you in danger. Knowing where
to look helps you to recognize hazards.
Workers face many hazards on the job:
• Inadequate wiring is dangerous.
• Exposed electrical parts are dangerous.
• Overhead powerlines are dangerous.
• Wires with bad insulation can give you a
shock.
• Electrical systems and tools that are not grounded or double- insulated are dangerous.
• Overloaded circuits are dangerous.
• Damaged power tools and equipment are electrical hazards.
• Using the wrong PPE is dangerous.
• Using the wrong tool is dangerous.
• Some on- site chemicals are harmful.
• Defective ladders and scaffolding are dangerous.
• Ladders that conduct electricity are dangerous.
• Electrical hazards can be made worse if the worker, location or equipment is wet.
Inadequate Wiring Hazards
An electrical hazard exists when the wire is too small a gauge
for the current it will carry. Normally, the circuit breaker in a cir-cuit
is matched to the wire size. However, in older wiring, branch
lines to permanent ceiling light fixtures could be wired with a
smaller gauge than the supply cable. Let’s say a light fixture is
replaced with another device that uses more current. The current
capacity ( ampacity) of the branch wire could be exceeded. When
a wire is too small for the current it is supposed to carry, the wire
will heat up. The heated wire could cause a fire. Again, keep in
mind and consider the following wiring hazards to ensure proper
safety:
29
Use the safety model to recognize, evaluate, and control workplace hazards
like those in this picture.
An electrician was removing a metal fish tape from a hole at the base of a metal light pole. ( A fish tape is used to pull
wire through a conduit run.) The fish tape became energized, electrocuting him. As a result of its inspection, OSHA
issued a citation for three serious violations of the agency’s construction standards. If the following OSHA require-ments
had been followed, this death could have been prevented.
• De- energize all circuits before beginning work.
• Always lock out and tag out de- energized equipment.
• Companies must train workers to recognize and avoid unsafe conditions associated with their work.
Worker was electrocuted while
removing energized fish tape.
Fish tape.
• Wire gauge— wire size or diameter ( technically, the cross- sectional area).
• Ampacity— the maximum amount of current a wire can carry safely without overheating.
• Overloaded wires get hot.
When you use an extension cord, the size of the wire you are placing into the circuit may be too small for the equip-ment.
The circuit breaker could be the right size for the circuit but not right for the smaller- gauge extension cord. A tool
plugged into the extension cord may use more current than the cord can handle without tripping the circuit breaker. The
wire will overheat and could cause a fire. The kind of metal used as a conductor can cause an electrical hazard. Special
care needs to be taken with aluminum wire. Since it is more brittle than copper, aluminum wire can crack and break more
easily. Connections with aluminum wire can become loose and oxidize if not made properly, creating heat or arcing. You
need to recognize that inadequate wiring is a hazard. Incorrect wiring practices can cause fires.
Exposed electrical parts hazards
Electrical hazards exist when wires or other electrical parts are exposed. Wires and
parts can be exposed if a cover is removed from a wiring or breaker box. The overhead
wires coming into a home may be exposed. Electrical terminals in motors, appliances and
electronic equipment may be exposed. Older equipment may have exposed electrical
parts. If you contact exposed live electrical parts, you will be shocked. You need to recog-nize
that an exposed electrical component is a hazard.
• If you touch live electrical parts, you will be shocked.
Overhead Power Line Hazards
Most people do not realize that overhead power lines are usually not insulated. More
than half of all electrocutions are caused by direct worker contact with energized power
lines. Power line workers must be especially aware of the dangers of overhead lines. In
the past, 80 percent of all lineman deaths were caused by contacting a live wire with a
bare hand. Due to such incidents, all linemen now wear special rubber gloves that protect
them up to 34,500 volts. Today, most electrocutions involving overhead power lines are
caused by failure to maintain proper work distances.
• Overhead power lines kill many workers!
Shocks and electrocutions occur where physical barriers are not in place to prevent contact with the wires. When dump
trucks, cranes, work platforms or other conductive materials ( such as pipes and ladders) contact overhead wires, the
equipment operator or other workers can be killed. If you do not maintain required clearance distances from power lines,
you can be shocked and killed. ( The minimum distance for voltages up to 50 kV is 10 feet. For voltages over 5 kV, the
minimum distance is 10 feet plus 4 inches for every 10 kV over 50 kV.) Never store materials and equipment under or
near overhead power lines. You need to recognize that overhead power lines are a hazard.
30
This hand- held sander has
exposed wires and should not
be used.
Watch out for exposed
electrical wires around
electronic equipment.
Electrical line workers
need special training and
equipment to work safely.
Operating a crane near overhead wires is
very hazardous.
Example of overhead power lines hazard
Defective Insulation Hazards
Insulation that is defective or inadequate is an electrical hazard. Usually, a plastic or rubber cover-ing
insulates wires. Insulation prevents conductors from coming in contact with each other. Insulation
also prevents conductors from coming in contact with people.
• Insulation— material that does not conduct electricity easily.
Extension cords may have damaged insulation. Sometimes the insulation inside an electrical tool or
appliance is damaged. When insulation is damaged, exposed metal parts may become energized if a
live wire inside touches them. Electric hand tools that are old, damaged or misused may have damaged
insulation inside. If you touch damaged power tools or other equipment, you will receive a shock. You
are more likely to receive a shock if the tool is not grounded or double- insulated. ( Double- insulated
tools have two insulation barriers and no exposed metal parts.) You need to recognize that defective
insulation is a hazard.
• If you touch a damaged live power tool, you will be shocked.
• A damaged live power tool that is not grounded or double insulated is very dangerous.
Improper Grounding Hazards
When an electrical system is not grounded properly, a hazard exists. The most common OSHA
electrical violation is improper grounding of equipment and circuitry. The metal parts of an electrical
wiring system that we touch ( switch plates, ceiling light fixtures, conduit, etc.) should be grounded
and at 0 volts. If the system is not grounded properly, these parts may become energized. Metal parts
of motors, appliances or electronics that are plugged into improperly grounded circuits may be energized. When a circuit
is not grounded properly, a hazard exists because unwanted voltage cannot be safely eliminated. If there is no safe path to
ground for fault currents, exposed metal parts in damaged appliances can become energized. Extension cords may not
provide a continuous path to ground because of a broken ground wire or plug. If you contact a defective electrical device
that is not grounded ( or grounded improperly), you will be shocked. You need to recognize that an improperly grounded
electrical system is a hazard.
• Fault current— any current that is not in its intended path.
• Ground potential— the voltage a grounded part should have; 0 volts relative to ground.
• If you touch a defective live component that is not grounded, you will be shocked.
Electrical systems are often grounded to metal water pipes that serve as a continuous path to ground. If plumbing is
used as a path to ground for fault current, all pipes must be made of conductive material ( a type of metal). Many electro-cutions
and fires occur because ( during renovation or repair) parts of metal plumbing are replaced with plastic pipe,
which does not conduct electricity. In these cases, the path to ground is interrupted by nonconductive material. A ground
31
Five workers were constructing a chain- link fence in front of a house, directly below a 7,200- volt energized power line.
As they prepared to install 21- foot sections of metal top rail on the fence, one of the workers picked up a section of rail
and held it up vertically. The rail contacted the 7,200- volt line, and the worker was electrocuted. Following inspection,
OSHA determined that the employee who was killed had never received any safety
training from his employer and no specific instruction on how to avoid the hazards
associated with overhead power lines.
In this case, the company failed to obey these regulations:
• Employers must train their workers to recognize and avoid unsafe conditions on the
job.
• Employers must not allow their workers to work near any part of an electrical circuit
UNLESS the circuit is de- energized ( shut off) and grounded, or guarded in such a
way that it cannot be contacted.
• Ground- fault protection must be provided at construction sites to guard against elec-trical
shock.
This extension
cord is damaged
and should not
be used.
fault circuit interrupter, or GFCI, is an inexpensive life- saver. GFCI’s detect any difference in current between the two cir-cuit
wires ( the black wires and white wires). This difference in current could happen when electrical equipment is not
working correctly, causing leakage current. If leakage current ( a ground fault) is detected in a GFCI- protected circuit, the
GFCI switches off the current in the circuit, protecting you from a dangerous shock. GFCI’s are set at about 5 mA and are
designed to protect workers from electrocution. GFCI’s are able to detect the loss of current resulting from leakage
through a person who is beginning to be shocked. If this situation occurs, the GFCI switches off the current in the circuit.
GFCI’s are different from circuit breakers because they detect leakage currents rather than overloads. Circuits with miss-ing,
damaged, or improperly wired GFCI’s may allow you to be shocked. You need to recognize that a circuit improperly
protected by a GFCI is a hazard.
• GFCI— ground fault circuit interrupter- a device that detects current leakage from a cir-cuit
to ground and shuts the current off.
• Leakage current— current that does not return through the intended path but instead
“ leaks” to ground.
• Ground fault��� a loss of current from a circuit to a ground connection.
Overload Hazards
Overloads in an electrical system are hazardous because they can produce heat or arcing. Wires and other components
in an electrical system or circuit have a maximum amount of current they can carry safely. If too many devices are
plugged into a circuit, the electrical current will heat the wires to a very high temperature. If any one tool uses too much
current, the wires will heat up.
• Overload— too much current in a circuit.
• An overload can lead to a fire or electrical shock.
The temperature of the wires can be high enough to cause a fire. If their insulation melts,
arcing may occur. Arcing can cause a fire in the area where the overload exists, even inside a
wall. In order to prevent too much current in a circuit, a circuit breaker or fuse is placed in the
circuit. If there is too much current in the circuit, the breaker “ trips” and opens like a switch. If
an overloaded circuit is equipped with a fuse, an internal part of the fuse melts, opening the
circuit. Both breakers and fuses do the same thing: open the circuit to shut off the electrical
current. If the breakers or fuses are too big for the wires they are supposed to protect, an over-load
in the circuit will not be detected and the current will not be shut off. Overloading leads to
overheating of circuit components ( including wires) and may cause a fire. You need to recognize that a circuit with
improper overcurrent protection devices— or one with no overcurrent protection devices at all— is a hazard.
• Circuit breaker— an overcurrent protection device that automatically shuts off the current in a circuit if an overload
occurs.
• Trip— the automatic opening ( turning off) of a circuit by a GFCI or circuit breaker.
• Fuse— an overcurrent protection device that has an internal part that melts and shuts off the current in a circuit if
there is an overload.
• Circuit breakers and fuses that are too big for the circuit are dangerous.
• Circuits without circuit breakers or fuses are dangerous.
Overcurrent protection devices are built into the wiring of some electric motors, tools and electronic devices. For
example, if a tool draws too much current or if it overheats, the current will be shut off from within the device itself.
Damaged tools can overheat and cause a fire. You need to recognize that a damaged tool is a hazard.
• Damaged power tools can cause overloads.
Wet Conditions Hazards
Working in wet conditions is hazardous because you may become an easy path for electrical current. If you touch a live
wire or other electrical component— and you are well- grounded because you are standing in even a small puddle of
water— you will receive a shock.
• Wet conditions are dangerous.
32
GFCI receptacle
Overloads are a major
cause of fires.
Damaged insulation, equipment or tools can expose you to live electrical parts. A damaged tool may not be grounded
properly, so the housing of the tool may be energized, causing you to receive a shock. Improperly grounded metal switch
plates and ceiling lights are especially hazardous in wet conditions. If you touch a live electrical component with an unin-sulated
hand tool, you are more likely to receive a shock when standing in water. But remember: you don’t have to be
standing in water to be electrocuted. Wet clothing, high humidity and perspiration also increase your chances of being
electrocuted. You need to recognize that all wet conditions are hazards.
• An electrical circuit in a damp place without a GFCI is dangerous. A GFCI reduces the danger.
Additional Hazards
In addition to electrical hazards, other types of hazards are present at job sites. Remember that
all of these hazards can be controlled.
• There may be chemical hazards. Solvents and
other substances may be poisonous or cause dis-ease.
• Frequent overhead work can cause tendinitis
( inflammation) in your shoulders.
• Intensive use of hand tools that involve force or
twisting can cause tendinitis of the hands, wrists
or elbows. Use of hand tools can also cause
carpal tunnel syndrome, which results when
nerves in the wrist are damaged by swelling ten-dons
or contracting muscles.
Examples of Additional Hazards ( Non- electrical)
• PPE— personal protective equipment ( eye protection, hard hat, special clothing, etc.)
• Low back pain can result from lifting objects the wrong way or carrying heavy loads of wire
or other material. Back pain can also occur as a result of injury from poor working surfaces
such as wet or slippery floors. Back pain is common, but it can be disabling and can affect
young individuals.
• Chips and particles flying from tools can injure your eyes. Wear eye protection.
• Falling objects can hit you. Wear a hard hat.
• Sharp tools and power equipment can cause cuts and other injuries. If you receive a shock,
you may react and be hurt by a tool.
33
Overhead work can
cause long- term
shoulder pain.
Frequent use of some hand tools can
cause wrist problems such as carpal
tunnel syndrome.
A 22- year- old carpenter’s apprentice was killed when he was struck in the head by a nail fired from a powder- actuated
nail gun ( a device that uses a gun powder cartridge to drive nails into concrete or steel). The nail gun operator fired
the gun while attempting to anchor a plywood concrete form, causing the nail to pass through the hollow form. The nail
traveled 27 feet before striking the victim. The nail gun operator had never received training on how to use the tool,
and none of the employees in the area was wearing PPE.
In another situation, two workers were building a wall while remodeling a house. One of the workers was killed when
he was struck by a nail fired from a powder- actuated nail gun. The tool operator who fired the nail was trying to attach
a piece of plywood to a wooden stud. But the nail shot though the plywood and stud, striking the victim. Below are
some OSHA regulations that should have been followed.
• Employees using powder- or pressure- actuated tools must be trained to use them safely.
• Employees who operate powder- or pressure- actuated tools must be trained to avoid firing into easily penetrated
materials ( like plywood).
• In areas where workers could be exposed to flying nails, appropriate PPE must be used.
Lift with your legs,
not your back!
• You can be injured or killed by falling from a ladder or scaffolding. If you receive a shock—
even a mild one— you may lose your balance and fall. Even without being shocked, you
could fall from a ladder or scaffolding.
• You expose yourself to hazards when you do not wear PPE.
All of these situations need to be recognized as hazards.
Summary
You need to be able to recognize that electrical shocks, fires, or falls result from these hazards:
• Inadequate wiring
• Exposed electrical parts
• Overhead powerlines
• Defective insulation
• Improper grounding
• Overloaded circuits
• Wet conditions
• Damaged tools and equipment
• Improper PPE
Safety Model Stage 2— Evaluating Hazards
How Do You Evaluate Your Risk?
After you recognize a hazard, your next step is to evaluate your risk from the hazard. Obviously, exposed wires should
be recognized as a hazard. If the exposed wires are 15 feet off the ground, your risk is low. However, if you are going to
be working on a roof near those same wires, your risk is high. The risk of shock is greater if you will be carrying metal
conduit that could touch the exposed wires. You must constantly evaluate your risk.
• Risk— the chance that injury or death will occur.
• Make the right decisions.
Combinations of hazards increase your risk. Improper grounding and a dam-aged
tool greatly increase your risk. Wet conditions combined with other hazards
also increase your risk. You will need to make decisions about the nature of haz-ards
in order to evaluate your risk and do the right thing to remain safe. There are
clues that electrical hazards exist. For example, if a GFCI keeps tripping while
you are using a power tool, there is a problem. Don’t keep resetting the GFCI and
continuing to work. You must evaluate the clue and decide what action should be
taken to control the hazard. There are a number of other conditions that indicate a
hazard.
• Short— a low- resistance path between a live wire and the ground, or
between wires at different voltages ( called a fault if the current is unintended).
• Tripped circuit breakers and blown fuses show that too much current is flowing in a circuit. This condition could be
due to several factors, such as malfunctioning equipment or a short between conductors. You need to determine the
cause in order to control the hazard.
• An electrical tool, appliance, wire or connection that feels warm may indicate too much current in the circuit or
equipment. You need to evaluate the situation and determine your risk.
• An extension cord that feels warm may indicate too much current for the wire size of the cord. You must decide
when action needs to be taken.
• A cable, fuse box or junction box that feels warm may indicate too much current in the circuits.
• A burning odor may indicate overheated insulation.
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You need to be espe-cially
careful when
working on scaffold-ing
or ladders.
Combinations of hazards increase risk.
• Worn, frayed or damaged insulation around any wire or other conductor is an electrical hazard because the conduc-tors
could be exposed. Contact with an exposed wire could cause a shock. Damaged insulation could cause a short,
leading to arcing or a fire. Inspect all insulation for scrapes and breaks. You need to evaluate the seriousness of any
damage you find and decide how to deal with the hazard.
• A GFCI that trips indicates there is current leakage from the circuit. First you must decide the probable cause of the
leakage by recognizing any contributing hazards. Then you must decide what action needs to be taken.
Summary
• Look for “ clues” that hazards are present.
• Evaluate the seriousness of hazards.
• Decide if you need to take action.
• Don’t ignore signs of trouble.
Safety Model Stage 3— Controlling Hazards: Safe Work Environment
How Do You Control Hazards?
In order to control hazards, you must first create a safe work environment, then work in a safe manner. Generally, it is
best to remove the hazards altogether and create an environment that is truly safe. When OSHA regulations and the NEC
are followed, safe work environments are created. But you never know when materials or equipment might fail. Prepare
yourself for the unexpected by using safe work practices. Use as many safeguards as possible. If one fails, another may
protect you from injury or death.
How Do You Create a Safe Work Environment?
A safe work environment is created by controlling contact with electrical voltages and the currents they can cause.
Electrical currents need to be controlled so they do not pass through the body. In addition to preventing shocks, a safe
work environment reduces the chance of fires, burns and falls. You need to guard against contact with electrical voltages
and control electrical currents in order to create a safe work environment. Make your environment safer by doing the fol-lowing:
• Treat all conductors— even “ de- energized” ones— as if they are energized until they are locked out and tagged.
• Lock out and tag out circuits and machines.
• Prevent overloaded wiring by using the right size and type of wire.
• Prevent exposure to live electrical parts by isolating them.
• Prevent exposure to live wires and parts by using insulation.
• Prevent shocking currents from electrical systems and tools by grounding them.
• Prevent shocking currents by using GFCIs.
• Prevent too much current in circuits by using overcurrent protection devices.
Lock Out and Tag Out Circuits and Equipment
Create a safe work environment by locking out and tagging out circuits and machines. Before working on a circuit, you
must turn off the power supply. Once the circuit has been shut off and de- energized, lock out the switchgear to the circuit
so the power cannot be turned back on inadvertently. Then tag out the circuit with an easy- to- see sign or label that lets
everyone know that you are working on the circuit. If you are working on or near machinery, you must lock out and tag
out the machinery to prevent startup. Before you begin work, you must test the circuit to make sure it is de- energized.
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Lockout/ Tagout Checklist
Lockout/ tagout is an essential safety procedure that protects work-ers
from injury while working on or near electrical circuits and equip-ment.
Lockout involves applying a physical lock to the power
source( s) of circuits and equipment after they have been shut off and
de- energized. The source is then tagged out with an easy- to- read tag
that alerts other workers in the area that a lock has been applied.
Disconnecting means required by Subjpart S must be capable of
accepting a lock ( consistent with 1910.147( c)( 2)( iii)).
In addition to protecting workers from electrical hazards, lockout/ tagout prevents contact with
operating equipment parts: blades, gears, shafts, presses, etc. When performing lockout/ tagout on
circuits and equipment, you can use the checklist items below as a guide.
• Identify all sources of electrical energy for the equipment or circuits in question.
• Disable backup energy sources such as generators and batteries.
• Identify all shutoffs for each energy source.
• Notify all personnel that equipment and circuitry must be shut off, locked out and tagged out. ( Simply turning a
switch off is NOT enough.)
• Shut off energy sources and lock switchgear in the OFF position. Each worker should apply his or her individual
lock. Do not give your key to anyone.
• Test equipment and circuitry to make sure they are de- energized. This must be done by an authorized person. ( An
“ authorized” person is defined as someone who has received required training on the hazards and on the construc-tion
and operation of equipment involved in a task.) ( See 1910.147( b) as applicable.)
• Deplete stored energy by bleeding, blocking, grounding, etc.
�� Apply a tag to alert other workers that an energy source or piece of equipment has been locked out.
• Make sure everyone is safe and accounted for before equipment and circuits are unlocked and turned back on. Note
that only an authorized person may determine when it is safe to re- energize circuits.
Control Inadequate Wiring Hazards
Electrical hazards result from using the wrong size or type of wire. You must control such hazards to create a safe work
environment. You must choose the right size wire for the amount of current expected in a circuit. The wire must be able to
handle the current safely. The wire’s insulation must be appropriate for the voltage and tough enough for the environment.
Connections need to be reliable and protected.
• Use the right size and type of wire.
• AWG— American Wire Gauge— a measure of wire size.
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Always test a circuit to make sure it is
de- energized before working on it. Lockout/ tagout saves lives.
Control Hazards of Fixed Wiring
The wiring methods and size of conductors used in a system depend on several factors:
• Intended use of the circuit system
• Building materials
• Size and distribution of electrical load
• Location of equipment ( such as underground burial)
• Environmental conditions ( such as dampness)
• Presence of corrosives
• Temperature extreme
Fixed, permanent wiring is better than extension cords, which can be misused and
damaged more easily. NEC requirements for fixed wiring should always be followed. A
variety of materials can be used in wiring applications, including nonmetallic sheathed
cable ( Romex ® ), armored cable, and metal and plastic conduit. The choice of wiring
material depends on the wiring environment and the need to support and protect wires.
• Fixed wiring— the permanent wiring installed in homes and other buildings.
Aluminum wire and connections should be handled with special care. Connections
made with aluminum wire can loosen due to heat expansion and oxidize if they are not
made properly. Loose or oxidized connections can create heat or arcing. Special clamps
and terminals are necessary to make proper connections using aluminum wire.
Antioxidant paste can be applied to connections to prevent oxidation.
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Wires come in different sizes. The maximum current each size can conduct safely is shown.
Nonmetallic sheathing helps
protect wires from damage.
Control Hazards of Flexible Wiring
Use Flexible Wiring Properly
Electrical cords supplement fixed wiring by providing the flexibility required for maintenance, portability, isolation
from vibration, and emergency and temporary power needs. Flexible wiring can be used for extension cords or power
supply cords. Power supply cords can be removable or permanently attached to the appliance.
• Flexible wiring— cables with insulated and stranded wire that bends easily.
DO NOT use flexible wiring in situations where frequent inspection would be difficult, where damage would be likely,
or where long- term electrical supply is needed. Flexible cords cannot be used as a substitute for the fixed wiring of a
structure. Flexible cords must not be:
• Run through holes in walls, ceilings or floors.
• Run through doorways, windows or similar openings ( unless physically protected).
• Attached to building surfaces ( except with a tension take- up device within 6 feet of the supply end).
• Hidden in walls, ceilings or floors.
• Hidden in conduit or other raceways.
Use the Right Extension Cord
The size of wire in an extension cord must be compatible with the amount of current the cord will be expected to carry.
The amount of current depends on the equipment plugged into the extension cord. Current ratings ( how much current a
device needs to operate) are often printed on the nameplate. If a power rating is given, it is necessary to divide the power
rating in watts by the voltage to find the current rating. For example, a 1,000- watt heater plugged into a 120- volt circuit
will need almost 10 amps of current. Let’s look at another example: A 1- horsepower electric motor uses electrical energy
at the rate of almost 750 watts, so it will need a minimum of about 7 amps of current on a 120- volt circuit. But electric
motors need additional current as they startup or if they stall, requiring up to 200 percent of the